Hi,
If it's for an alarm system, aiming to sense if someone is tampering with the car you could use an acclereometer. Or perhaps something from these guys:
http://www.assemtech.co.uk/assemvibration.asp Or a simple steel ball suspended in a wire hanging inside a tube. When the car moves the ball starts swinging and makes contact with the surrounding tube - trig whatever... (Like the old pinnball machines tilt sensors)

If it's not for an alarm system then I'm way off (except for the accelerometer)

HTH
/Henrik Olsson.