I got my GM Gentex 13581081 Auto-Dimming mirror in the mail yesterday, and had a lot of fun retrofitting it to my base model 2013 Volt. I can do a full writeup if it's warranted, but I'm not completely done yet.
What I have been able to accomplish:
Ignition Switched 12v power
Connection to ground
Factory fit and finish
What I have yet to accomplish:
Side mirror retrofit (not in the budget yet)
Automatically clear dimming while reversing.
Getting the mirror mechanically installed was easy, so was locating the proper power and ground. The default location of the mirror mounting button is perfect for both the manual and automatic mirrors (which makes sense. Why would GM make extra work for themselves?). Getting power was a little more of a pain in the behind. I thought GM would have used the same wiring harness for the base model and the Premium trim level and just tucked the mirror connector into the headliner or taped it off somewhere, but alas, it was nowhere to be seen. I ended up tapping off the Black and Yellow leads to the garage door opener, which were conveniently exactly what I expected them to be. (Black ground, Yellow Ignition Switched 12v+)
The socket on the mirror had to go, since I was using my own wires, but here's the pinout in case anyone's interested:
  [__] [__] 
[__] [__]   [__]
1 - Dark Green - Reverse Sense
2 - Red - 12v+ Input
8 - Black - Chassis Ground
9 - Purple - Side Mirror
5 - Pink - Side Mirror
(The mirror colors might be backwards. I'm going from memory)
Like I said, I ultimately ended up just clipping the modular connector off and soldering my own leads on with bullet connectors. Green is reverse sense (takes 12v+ to trigger. Tie it to ground if you don't want to use it, don't just leave it snipped) Red is 12v+ power, Black is chassis ground, and the remaining two wires are there to control auto-dimming side mirrors. I still haven't decided where I'm going to tap the reverse light, so I haven't connected that up yet. I'm tempted to route it all the way up the body so I have an easy 12v+ rail that's switched with the reverse light, since I also have a reverse camera on its way, but I'm just as tempted to stick a switch on a box or put a touch strip on the back of the mirror that activates a relay and 12v for a manual override.
For the curious who want to try this, the overhead button panel is secured with panel clips. They're little metal-reinforced clips stuck to plastic tabs that stick up into the headliner. There's no way to pre-compress them to make removing the panel easier. It just takes patience and a somewhat frightening amount of force. I believe there are 6 of them; one for each corner, and two in the middle on the sides. They look like this:
(Image attached in case jetpress changes the page or product, or if they remove or rename the page)
There is a hole in the headliner right next to where the keyfob and TPMS receiver's cables route that's aligned and designed to pass power and signals to and from advanced mirrors like the auto-dimmer and other possible options. Glad to see GM didn't remove all traces of an upgrade path.