I have to clean mine up a little this weekend, but good for now..
I pulled the mirror out of the housing (very easy, just push it downward - aiming at the ground - and gently pull it off from the backside.
Then I used a clothes-hanger wire to fish my wiring through.
I removed the speaker door so I could reach in and work with the wire, and brought it out through a hole (in the door) just above the speaker. There is also a hole in the body of the car just below the GM wire harness that I used to run the wire into the car.
To get into the cabin (on the V6 at least), there is a plug just below the battery wire at the firewall(you probably can't see it, but you can feel it). I popped it out, and went in that way to get to the front blinkers.
Again, not the cleanest look for now, but the easiest route for sure (without drilling or cutting anything).