i've heard about systems that just hook up a second car battery to the car's sheet metal (on/off by remote). if anyone brushes up against the car, they get shocked.

is there a liability if your car device injures or kills the potential thief? i'm glad we live in Canada where would be criminals can't be nearly as litigous as in the states, but then again, we dont' live in Texas either.

http://www.rosholt-international.bigstep.com/autotaser_files/autotaser.html

this looks a bit better: taser technology is high volts, low current so it doesn't kill the guy, it only incopacitates him. maybe you could rig someething integrated with a hand taser on the 12v line, connected to some conductive material either by the ignition or the steering wheel.

i'm thinking you'd face attempted/murder charges if you electricuted the would be carthief, becuase i think we have "escalating force/response" laws in Canada. if you really want to be sure, sparkplug in the gas tank (: