most of them (at least on older models) just since a ground signal from the Parking break. On my caliber radio i grounded the parking sensor wire and it deactivated the part sensor. so i would look for the signal wire and see if its just a ground.
i just had an aftermarket one installed; basically one of the wires goes to the battery and one goes to a constant ground. very easy work around when you find the right wires in the back of the unit so you can play your dvds, music videos, and navi