Depending on what type of rear view mirror you got, there may be two or three PG wires.
If you have the type with HomeLink + Antiglare/Compass, then, there are three wires:
Black: ground
+ two Power wires, one is constant 12V, and the other is switched 12V.
Use a multi-meter to tell which is which.
I would like to remind you that, the rear view mirror's PG wires tap into existing wires.
If you tap into them again, you might overload the fuse. Better do some calculation before you blow the fuse.
P.S. HomeLink needs constant 12V (so you can close/open garage doors w/o turning the ignition).
Antiglare and compass use the switched 12V to save power since they don't have to function if the vehicle is not powered on.