I'm in favor of the switched cigarette lighter.
Wire gauge is appropriate
You already own it!
Also look for a factory wired cd changer sometimes you can grab those wires for your evil purposes. My VW has more than a few (unused) wires running from the dash to the trunk.
To control the relay I've been using the power lead to the taillight. It gives me an audible alarm if I leave it on when exiting the car and allows me to turn on the pc for short periods with out starting the car.