TexasCHLforum.com

Years ago, a friend of mine from back east had a problem with "uninvited sitters" sitting on the seat of his motorcycle all the time. The bike was a collector's item... ...an old Indian or something like that, and it was pretty cool. However, a lot of times when he would take it out for a ride and make a stop at a roadhouse or someplace like that, when he came back out there would be somebody sitting on the seat of his bike, checking it out, without owner's (his) permission.

So, he rigged a small capacitor under the seat that was deactivated by inserting the key into the ignition and starting the motorcycle. If you sat on the seat first without starting the bike, it would deliver a Louisville Slugger sized swat of electrons to your hind parts. It only had to happen a couple of times before word got out not to sit on my friend's bike without his permission.

I'm only telling this story to suggest that maybe you could do something similar to your truck's tailgate?