ElectroChromic Mirror (ECM) Installation

The ECM has light sensors to help decide when to automatically dim the mirror. More importantly, it has 3 Homelink buttons that can be programmed to actuate garage doors, community street gates, lights, home deadbolts, etc (see Homelink's site for details on who has Homelink compatible items).

If you ordered your 2004 Prius without a package containing the ECM, and did not have the port install it, you can do it yourself, or have a local shop install it.

On a difficulty scale, I rated this installation as a 3/5 only because of the amount of interior trim that has to be removed. There still aren't any permanent mods to the car, so it is easily reversible.

    The unassuming box that the ECM comes in. With the ominous sticker on it that says:

        The installation instructions for
        this accessory are on the
        electronic Toyota Information
        System (TIS). Please see your
        Toyota dealer for a copy.

    Nice of them to not include install instructions. Luckily, we have the install pdf in the Prius-2G Files/Manuals section if you need it. You shouldn't need it after reading this.


    Open the box, and you have the ECM wrapped in a foam sheet, and the hardware install bag.


    As seen, the install bag contains (L-R):

    • wireties (for fastening the ECM wiring harness to the OEM harness behind the A-pillar)
    • wiring harness (10-pin plug, 3 wires)
    • T-taps (for tapping stock wires to power the ECM)
    • windshield wire cover (to cover the wire from the ECM to the headliner)
    • adhesive foam squares (for noise/vibration control/mounting for the wiring harness)
    Not shown is the "Operation Instruction Card" which details testing the dimming function.


    I strongly recommend you get a set of autobody bodyfiller spatulas for trim removal. They are soft plastic, so they won't scratch the interior pieces, but will give you leverage for popping panels loose. The ones pictured were a whopping $1.24 at Walmart. You should also be able to get them from NAPA or any autoparts store.


    Remove the trunk floor, then and remove the subfloor cargo box by lifting at the circled lift areas.


    Remove the right side battery cover. Pull the hard plastic rectangular bit away from the rear wall as shown, then lift it to remove the cover. Note that the curved front tab will hold it in place unless you lift up while pulling to the rear.


    Disconnect the negative side of the auxiliary battery. You can either fight with the bolt/nut on the negative battery terminal, or just remove the cable end from the body as shown. If you remove the cable end, make sure to put it in a plastic bag or wrap it with electrical tape or something else so it can't contact the body and reestablish ground. If you fight with the bolt/nut on the terminal, do yourself a favor and turn the bolt around in the terminal to make loosening the nut easier in the future.

    At this point, do not close the hatch until the entire procedure is done, or you won't be able to open the hatch to reconnect the battery (or will be putting the back seats down to access the manual release for the hatch).


    Remove the driver side vent panel. This takes some pulling or prying to get it to start coming loose. Note the junction of the top edge of the vent and the main dash panel. Wedge a spreader as far into that junction as possible, then press on the front side (towards the windshield) down near the junction, pushing the vent panel straight back away from the dash. The top clips will pop loose (the first time they may take a good bit of force).


    Once the top of the vent panel is loose, slide your fingers down the panel as far as you can, and pull the panel straight back. The clips holding it at this point are very near the bottom of the panel.


    Remove the 2 screws securing the lower dash cover, then pull straight back around the edges of the lower cover to pop it loose.


    Press the tab on the dash rheostat plug firmly to unplug it.


    Unplug the 2 plugs on the keyfob slot.


    Unplug the Smart Entry/Smart Start button, if your car is so equipped.


    Remove the hood release latch from the dash panel - depress the circled tab, then pull the latch away from the panel.


    Remove the hood release cable from the latch - grasp the cable, and pull the latch off of it (it will trip the hood, so make sure to latch the hood later).


    Turn the latch over, rotate it so the cable lines up with the notch shown, and pull the cable sideways out of the latch. Set the latch and the lower dash cover aside.


    Be careful when handling the lower dash cover, as the insulators (circled) break easily.


    Slip your fingers under the upper dash cover and pull straight back to pop the clip loose, then work towards the right side to pop the right top clips loose..


    Grab under the bottom right corner of the top dash cover, and pull straight back to finish pulling it loose.


    Rotate the bottom of the top dash cover up, then swing the cover to the passenger side until you can flip it over and unplug the Power button. I'd also recommend pressing in the metal tabs on each side of the Power button and removing it from the dash cover (explained later). Set the Power button and dash cover aside.


    Remove the 3 10mm nuts holding the metal plate over the junction box, and remove the plate. This finishes the dash work for now.


    Insert a spreader between the rearview mirror base and windshield and pry the cover off of the mirror base. It takes very little force to pop it loose.


    Push the cover down the mirror arm to get it out of the way. Pull down on the black plastic tab (arrow) holding the mirror to the mirror button, slide the mirror up towards the headliner, and remove it. Store it somewhere safe in the ECM box in case you need it in the future.


    Open the sunglass holder in the overhead console, and remove the 2 Philips screws noted. Store them in a separate labeled plastic bag - these screws are very short, and if you use longer screws in their place, you risk screwing through the roof of the car!


    Slip a spreader between the console and headliner, being careful not to mark anything. Pull down just enough to get a fingerhold.


    Wedge your fingers in that corner as best you can, and pull directly down on the console. You have to pull down with some force to pop the connector loose. Once that corner is loose, move to the right rear corner.


    Slide your fingers to the right side front, close to the front edge, and pull straight down to pop that connector loose.


    Repeat on the front left corner, and this is what you should see...maybe. Note that one of the white connectors stayed in the roof frame.


    If a connector stays behind, simply grab it with a needlenose pliers and pull it out. Note that it is best to get the pliers securely gripping the connector, then place you fingers behind the pivot point of the plier jaws, and push on the plier handles to lift the connector out.


    Unplug this connector on the console, and set it aside.


    Pull the drivers sun visor out of its' clip. Grab the clip with your fingers, and turn it counterclockwise (CCW) exactly 90 degrees. Do NOT turn it clockwise, or turn it CCW more than 90 degrees, or you may break its mount.


    Remove the 2 screws holding the driver's sunvisor on. Make sure to support the visor as you start removing the second screw.


    This is why you support the visor while removing the second screw - you don't want this harness to be pulled around...yet.


    Carefully work the white plug and several inches of the visor harness out of the roof. Depress the tab on the headliner side of the plugs (inset) to release the visor. Set it aside.


    Slip as much of the spreader between the headliner and driver's A-pillar trim as possible, put your fingers behind it, and pull downward directed towards the center seat console.


    The A-pillar will break loose, but then stop after the top is 1" from the headliner. There is a hooked fitting that needs to be worked loose.


    This hooked top fitting needs to be grabbed with a pair of very long needlenose pliers, forceps, etc, and twisted 90 until the hook aligns with the slot in the A-pillar. Once it aligns with the slot, pull the A-pillar off of the hook.


    Slip the spreader between the main dash cover and the A-pillar insert, and twist it, so that it is lifting the back edge of the insert up.


    Once the rear is up, pull the insert towards the rear of the car to pull the front tab loose.


    Removing the A-pillar insert makes removing the A-pillar much easier. Once it is out, lift the A-pillar up and towards the center of the car, wiggling as you lift, and it should come right out. Put the A-pillar insert back in the A-pillar, and set the A-pillar aside.


    Slide the ECM onto the mirror mounting button on the windshield, and tighten the mounting screw (Torx screw) to 15.5 in-lbf if you have a small torque wrench. If not, tighten it snugly.


    Plug the ECM wiring harness into the back of the ECM.


    Route the wiring harness into the groove in the windshield wire cover and attach the windshield wire cover to the mirror mounting bracket..


    This is best done by temporarily routing the harness up into the headliner a bit to keep it out of the way and somewhat aligned with the installed position of the wire cover. Extend the windshield wire cover until its tabs fit firmly under the headliner, being careful not to bend the headliner.


    Route the wiring harness into the headliner. Place the harness under the headliner padding and down alongside the factory A-pillar harness. Make sure you leave enough harness slack near the mirror so that you can move the mirror full left/right/up/down without pulling the wire tight. It is easiest to route the wire down the A-pillar by very loosely fastening wireties as you go.


    Cut 3 pieces of foam tape in half on the long side, and use 5 of the pieces as shown. Be sure to place the foam tape under the sharp metal clip when taping to the headliner.


    Near the mirror. Note that the original instructions almost make it sound like you will be taping the harness to the headliner, but I taped it to the roof.


    The "sharp metal clip". A cut an extra ½ piece of tape and taped it down right under this clip. There are extra pieces of foam tape in the install kit, so you can do this.


    As I worked my way towards the A-pillar, I was losing flexibility in the headliner to place the foam tape. So I removed the grab handle above the driver's window to gain more working space. I removed the covers/spreaders over each base by prying in the tiny slots on each side, then wiggling the covers/spreaders out. Then I squeezed the 2 metal tabs towards each other and pulled the grab handle free (getting it back on was a bit more difficult, I had to hold the metal tab with needlenose pliers as I placed the grab handle over it, to keep it in place). With the grab handle removed, I was able to wiggle the headliner past the grab handle mounting tabs, and gain an extra 1-2" of space to work in.


    I secured the mirror wire harness to the OEM wire harness with extra wireties down near the windshield base - I recommend you get some extre wireties and do the same.


    Rather than fasten the harness to the AC duct as noted in the original instructions, I cut 2 foam tape pieces, and taped it down to the body in front of and behind the AC duct.


    Cut another piece of foam tape in half on the long side, and use one piece to secure the mirror wiring harness to the top of the AC duct, and the other piece to secure it to the bottom of the AC duct. There's no easy way to picture these areas, so I've included the diagram from the instructions.


    The harness has 3 wires:

    • The ground wire, with a ring terminal installed
    • Black wire (B+) with male spade terminal
    • Black/white wire (ACC) with male spade terminal
    Pull these wires apart from each other until you have 8" of each free from the others. Wrap the black and black/white wire with electrical tape.

    Locate the 36-pin white connector on the front of the I/P junction block.


    Locate the violet wire on pin #19 from the connector, and apply a T-tap to it about 1 inch from the connector.


    Lay the violet wire into the T-tap, close the T-tap, then squeeze it closed with pliers. Make sure the T-tap halves are snapped together securely.


    Plug the black/white wire into this T-tap. Make sure the spade terminal is inserting correctly into the middle of the T-tap end, so it makes contact with the internal terminal.


    Locate the 6-pin white connector on the front of the I/P junction block.


    Locate the red wire on pin #5 from the connector, and apply a T-tap to it about 1 inch from the connector. Make sure the T-tap halves are snapped together securely. Plug the black wire into this T-tap. Make sure the spade terminal is inserting correctly into the middle of the T-tap end, so it makes contact with the internal terminal.


    Locate the vehicle ground bolt (10mm) on the IP brace, secure the ground wire to the OEM harness with 1 wire tie, remove the bolt, place the ring terminal of the ground wire on the stud, and tighten the nut back down.


    Bundle any excess ECM harness to the vehicle main harnesses and secure with wireties (there was very little excess, as shown).


    At this point, according to the original instructions, you need to partially reassemble the car to run tests. I did it slightly differently, so that I could test while the car was still completely apart, so that if anything was wrong, I had full access to the job. I plugged the Power button into it's plug (this is why I removed it from the top dash cover earlier), then connected the battery cable back to the 12V battery. This was enough to run the tests, while not throwing any ECU errors, as anything unplugged was non-critical.

    With the vehicle in a fairly well-lit area, perform the Auto Dimming test:

    1. Power the car up.
    2. Make sure the auto-dimming feature is on - verify that the green LED to the left of the center switch is on. If not, push the button to the right of the center button to turn it on (the one marked "|").
    3. Cover the forward-looking photocell (the one on the back of the mirror, facing the windshield) with a dark cloth, your finger, etc. It is located above the wiring plug.
    4. After a few seconds, the mirror should begin to darken (the time may vary depending upon ambient light levels).
    5. Remove the cover from the photocell, and the mirror should start to clear.
    6. If you are doing the install indoors, so the mirror is already darkening without touching either photocell, try shining a flashlight into the front-facing photocell to see if the mirror clears - this is how I tested it in the garage with the garage door open.


    Perform the Homelink test:

    1. Power the car up.
    2. Push each Homelink button, one at a time, and verify that the LED Indicator light is red as each button is pressed.
    3. Power the car down.
    4. Push each Homelink button, one at a time, and verify that the LED Indicator light is red as each button is pressed.


If everything is working properly, now you can finish the assembly:

  1. Tighten all wireties and trim the ends off.
  2. Reinstall the metal plate covering the I/P junction block.
  3. Unplug the Power button, push it back into the upper dash cover, plug the Power button back in, and clip the upper dash cover into place.
  4. Reconnect any other harnesses/plugs you may have disconnected from the lower dash cover, and reinstall it, using the correct screws.
  5. Reinstall the A-pillar trim - push the bottom of the A-pillar into place, wiggling it around until it is seated, then swing it up near the "twist fitting". Twist the fitting 90 degrees, push the A-pillar over it, then release the fitting so that it locks into the A-pillar.
  6. Press the vent panel back into place.
  7. Plug the sun visor back in, screw it back down, and install the visor clip (90 CW this time)
  8. Plug the overhead console back in, line the pins up with the holes in the roof, and push it into place. Screw it down in the sunglass holder area. Make sure to use the proper screws here!
  9. Reinstall the battery cover, luggage box, and trunk floor.
  10. Place the Operation Instruction Card in with your other manuals in the lower glovebox.
  11. Make sure the hood is latched tight.


You will also need to check a couple of other things due to the battery disconnection. From the Owner's Manual:
If the auxiliary battery is disconnected or run down, the back door (hatch) will be automatically locked and the back door system may not work after you reconnect. Normalize the back door system by the following procedure:

  1. Unlock the back door with the remote control key or power door lock switch.
  2. Close the back door completely by hand.
If the auxiliary battery is disconnected or run down, the power window may not operate automatically and the jam protection will not function correctly after you reconnect. To normalize the power window:
  1. Push down on the power window switch and lower the window halfway.
  2. Pull up on the switch until the until the window closes and hold the switch for a second.
  3. Make sure the window opens and closes automatically.


Because the ECM has light sensors on the front and rear of the mirror to help decide when to dim the mirror, if yours does not seem to be dimming enough, try covering the front sensor partially with a piece of tape.

Legalese: Do these fixes/mods at your own risk. I have had no problems completing the ones that I authored, but cannot take responsibility for others' FAQs, nor for anyone screwing up a procedure listed here.
This document can be reprinted for personal use or reference for your mechanic(s) with no prior permission needed. It can be linked to directly, as long as you have a link to the main VFAQ page of http://www.vfaq.net/, though linking to the main page instead is preferred. This document can NOT be reprinted for profit/resale/redistribution of ANY type without expressed WRITTEN permission from me in advance. Bulk copying of this document onto your web site without prior permission will not be tolerated, link to it instead.
First uploaded: Apr 7, 2004
Last modified: Apr 17, 2004
Copyright 2004, Tom Stangl