Make a theremin and use the whole body car as antenna. A potential thief will be rapidly dissuaded for a frightening rising tone of increasing volume and pitch as he/she approaches to the protected car.
Optional: play along the sountrack of the 'Psycho' shower scene-- piluso, Feb 26 2016 Was this not what theremins were originally invented for?-- mitxela, Feb 26 2016 I think originally they were wired to the refrigerator for use as a dieting aid.-- whatrock, Feb 26 2016 Good, especially if it takes the form of a bent coat hanger protruding from where the aerial would normally be located.-- xenzag, Feb 27 2016 The would-be thief, once he figured out what was going on, would become so distracted trying to play "Lady of Spain" that he would be easily apprehended.-- smendler, Feb 28 2016 random, halfbakery