I'm literally in the middle of this little task for the 4-wire mirror and you knocked out this excellent write-up 20 minutes before I searched the forums for an answer. You're my hero.Not sure if you figured it out but thought I'd share since I just got this same mirror.
This is how I wired everything and it is working great:
Black wire in the mirror harness is Ground. Splice it to black wire in the overhead light harness.
Greenb were in the mirror harness is your constant 12v (Homelink). Splice it to the yellow wire in the overhead light harness.
White white on the mirror harness is the intermittent 12v or when keyed on (Compass). Splice it to the purple wire in the interested harness.
This is what I did and it works as I want it to. The garage buttons work always. The compass comes on when I turn the key to the on position. I know some prefer to have the garage buttons to work only when keyed for safety. But I kept a garage door opener in the truck previously anyway so what is the difference? It's much more convenient to just always be available. I love this mod. About $21 shipped and works perfectly. Took about 15 minutes to complete.
I will add that I have a Sommer Direct Drive garage door opener. The Homelink instructions included with the mirror to program it doesn't work. It was very frustrating to be following the steps. I found a PDF of the instruction manual online which had different instructions that worked perfectly:
Hold down all three garage door buttons until the lights turn off. Takes approximately 30 seconds. You can verify the garage buttons are in learn mode by pressing them. They should rapidly flash for 2 seconds and then turn solid. Now that Homelink is in learn mode press the learn button on the garage door opener and then go back to your mirror and press and hold the garage door button you wish to pair for approximately 3 seconds. It will rapidly flash and then turn solid. Now press the garage door button you just programmed and it should work. All other methods online got Homelink did not work except for this one.
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