If I had my druthers, I would rather have the OnStar mirror mount. It makes perfect sense that the mirror is the better location for it. But I am not going to go though lots of rewiring to move this minor annoyance.
The annoyance? ah yeah, well, my dogs will generally sit up in the front seat area when I don't have a passenger with me (like my wife). So they like to get comfortable and sometimes place their head in the area between the gear shifter and the OnStar button. A few times now I'd be driving down the road and the "OnStar Ready" voice comes on. Seems that the dog’s nose easily presses that darn button.
Oh yeah, that brings up another annoyance. The other location that the dogs will rest their head, when in the front seat, is to put their head on the passenger door arm rest. Exactly where the door lock button is. So, what I did was take a small rectangular piece of black plastic, exactly the outside size and shape of the door lock button, and I glued it there. It has no top, it is just a small 4-sided ridge surrounding the buttons. So a person can easily press the buttons to lock or unlock the doors. However the dogs head will not activate the button. Works for me.