I think some people have wired into the OnStar module near the mirror? Potentially easier/closer.
I used this method. This way the cam is separately fused so if it shorts out or something, I'm not going to take out another piece of gear with it.I've always just popped off the a-pillar trim and run the wiring for different things (cameras, radar detectors, etc.) underneath. There's usually an easy way to then feed the wire below the dash from the front corner of the windshield - no need to mess with any rubber trim or anything around the door.
Just ground the negative wire to any good body nut/bolt using a circular wire connection. You can buy a small device at most auto parts stores that will allow you to put two fuses in one fuse slot so the original fuse is still there plus a separate fuse for your new circuit. I would use something that is switched with the car power such as the radio or some other low power fuse.
The important thing is to make sure you use zip ties or something to secure your wiring BEHIND the side curtain airbag which is rolled up inside the pillar. You want to ensure that if the airbag goes off it doesn't propel your wiring into your head or something else equally bad.