I am Looking into it the exact same thing but haven't pulled the trigger yet. I found the pinout of the 16 pin mirror connector over on
www.gm-trucks.com. Info is below.
https://www.gm-trucks.com/forums/topic/51376-rearview-mirror-wiring-pinouts/
used
2 - Microphone (if the mirror is equipped with it)
3 - "Passenger Air Bag on" notification (if mirror is equipped with that display)
4. - "Passenger Air Bag off" notification (if mirror is equipped with that display)
5 - auto day/night signal, which comes from the LH outside rearview mirror
6 - Temp probe connection ( if mirror is equipped with display)
7 - Temp probe connection (if mirror is equipped with display)
8 - Ground
9 - one source says it's a Reverse Dim Inhibit signal so the mirror will not dim when
the vehicle is in reverse, another says it's a supply voltage from a back-up supply
(I guess so Onstar will still work in the event that the main evehicle battery is disabled in an accident).
I'm pretty sure if 12v is applied to this pin, the mirror will not dim no matter what.
10 - Microphone (if mirror is equipped)
11- keypad signal (out) for On-star (pressing any button puts out a 5v multiplexed signal on this pin,
- so you can't just use the buttons to control whatever you want to. Sorry.)
12 - keypad supply (5v) voltage (in) for the buttons
13 - ignition voltage (12v -main power IN to the mirror)
14 - keypad green LED signal
15 - keypad red LED signal
16 - return line to the LH outside mirror for day/night signal