I like the Trunk Monkey idea too, but you have to feed them.
I'm intrigued by the possibility of using old cell phones, hidden in the truck and hooked up to a permanent charging cable, that are set up with either MacroPoint or this phone app:
Timed FX , for just the sort of problem you had with theft.
There must be other solutions that are meant for permanent mounting that would even alert you to initial movement.