A relay set up off of your ignition would be simple. It will turn on and off when you turn the car on and off. The down side is it will be on the entire time the ignition is on.
Another thought would be to use a relay and have the relay triggered by temperature instead. This way it will only come on when needed.
i think im going to put a label in the cab that says "TURN ON YOUR FAN JACKASS!"
