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I mounted mine in the same place. I pushed the wire up under the headliner, and down the driver side A-pillar. Plenty of room under the molding to install a few butt connectors.
Remove the molding by removing the screw cover, then pull the molding off.
If you have a sunroof, there will be a yellow wire, this is through the RAP relay, splice into that one for power. Find a black wire for ground. (Don't forget to stagger the butt connectors to prevent any clearance issue when installing the molding.)
Vanity mirror power supply also runs up this pillar, they are powered from the BCM through the inadvertent power supply circuit (if left on, the BCM will shut off power after twenty minutes unless vehicle is running or a switch input is sensed). This would be an orange wire.
If you have no sunroof, or vanity mirrors, you do have an electric rearview mirror.
The dark blue/white wire powers the rearview mirror and is hot in start and run, via the RUN/CRNK relay. In this case, the ground would be black/white.
When tapping into the black ground wire, make sure to tap into the thickest of the black wires, in fact, it is a good idea to turn off the vehicle, and check continuity of the black wire to ground.
Onstar also has some wires going through the pillar and one is a black wire use for Keypad supply voltage. In addition, there are brown/white, gray, dark blue wires that are used for Onstar keypad signals.