it kind of depends on what your trying to do with it. as far as i know, all the power in the car is 12v (any of the fuses etc) but you could pull 5v off a head unit with usb (very low amperage ratings) or a cigarette lighter usb plug. otherwise your going to have to find a DC-DC power supply that goes 12 to 5v.
Or if your electronically inclined you could just make one yourself, its really as simple as getting a step down transformer, a resistor and a stiffening cap.
