if you want an alarm, just buy a car alarm and install it.
if you are looking for a kill switch, I prefer to wire it in (with a relay) to the pump resistor wire so the truck starts but dies after each attempt making it appear that it's just an old piece of crap rather than a protected truck where the thief just needs to find the kill switch. this is hypothetically speaking of course. I've got no idea of the thought process inside a thief's head.