The mic for OnStar is on the bottom side of the mirror. You can see the vent holes for it of you look straight up at the mirror.
But it would be easier to grab the audio in/out at the OnStar unit itself. Which is on the passenger side trunk behind the carpet.
Personally I would grab the audio in/out and leave the harness plugged into the OnStar. So when you press the OnStar button, the car will slow down the HVAC fan and what not when a call is in progress.