Guido, mate of mine did that about 35 years ago, got fed up with local kids/scroats bending the aerial on his old MK11 Jag. Wired up a trembla coil to an ignition coil and all the external metal bits, anyone touched his car got 40,000 volts up their arm. Local copper told him to remove it otherwise he would have to nick him, said it could give some old lady a heart attack if she touched it. In South Africa they have "anti hijack" devices that fire flames out from under the car doors-now there's an idea.
John