While there may be 5 V sources in the vehicle, I suggest not messing with them. You screw them up, the engine won't run.
A simple +12 to +5 volt supply can be made with a 7805 (or LM340T-5) IC and a couple of capacitors for noise and voltage stabilization. A lot depends on how much load current you need to draw and what the actual load device is.
Steve
