[notfarnow]

Of all the vehicles I've driven & owned, I've only had one attempted theft... on my suzuki samurai of all things.

Funny thing is that I lost the keys about 9 months ago, and wired a hidden switch to start it. The ignition switch itself is not connected to anything... the perp just gave up when he couldn't figure it out

Good trick for a kill switch:
Put a switch to send the power from the fuel pump to the horn. Bad enough if the vehile won't start, even worse if the horn goes off

A step further:
Instead of sending the power directly to the horn, run it through a relay, but loop the "switched" + back into the + switch feed.

That way, when the horn gets power, it also sends + back to the relay to keep it ON. Put a hidden rest button to break the "loop".

Very effective for ~$20