Simple Electrical question

If you want it to turn on with your car, I'd tap into the accesories line (not sure where it is though), turns on your radio and what not when the car is turned on.

As for the fuse, what amperage is the device rated for? Should tell you how much current it draws, then go and find a fuse that's a tad bigger than that amount. A fuse never hurts and has saved me quite often.

See http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif for help with the wiring