This install was on a 2009 Base.
I got the mirror out of an old Cadillac at the Junkyard. Many Cadillacs and Buicks have the autodimming mirror as standard equipment, in case you're looking.
Because I'm tall, I like to have my mirror as far up as it will go so I can see under it. While newer versions of this mirror have the compass electronics inside, this early version of the Gentex mirror with compass has the compass electronics external to the mirror, on a fixed cable. Because I wasn't able to use the factory intended mounting on the top of the mirror arm, I simply cut excess plastic of the compass housing intended for mounting to the mirror arm, to make it as small as possible, and then mounted it in the proper orientation (cable facing towards front of vehicle) with some 3M VHB tape.
The 16 pin plug has all the wires needed except for 12V for the autodimming mirror. I used a short length of cable from a donor plug I also got at the junkyard and inserted it at position 13. I soldered this wire to an extension and ran it along just inside the headliner, down the A pillar to the same power connection that I use for my dashcam, so it's switched with the ignition. Onstar still works as far as I can tell.
The compass works fine, although it needs to be calibrated first. Hold down the power button until "C" is displayed, then drive around in circles in an empty parking lot to calibrate. A quick press of the button toggles the compass display on or off. A longer press toggles the autodimming mirror on or off. More instructions are here: http://www.gentex.com/sites/default/fil ... ooting.pdf
You can also set the compass zone according to maps in this PDF.