With stuff like that where there's an on/off switch on the unit, I usually go through the Cigarrette lighter. It's really easy to get access to and I hardly ever use the cig. lighter for anything else anyways.
Besides that, anything like the radio that comes on when the vehicle is "on" will work great...or anything that is always hot, no matter if the vehicle is on or off.
Just avoid anything like the headlights, heater, interior lights, ect that only comes on when you turn them on....which would be a real pain.