Friend of mine used to know a mechanic who did something cool. He rigged up a little circuit so that unless you pressed some inconsequential small button on the car (I think it was the aircon) 5 times within 5 seconds of starting the engine it'd turn off again. Very simple security, but it'd take a thief hours to work out.
That's what a kill switch is. The simplest one is just a toggle switch to the ignition system that you hide somewhere non-obvious. Almost no thief would bother looking for it as that could take hours to find.
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