My old man before he retired was an auto elec and wired his boat trailer to his car alarm. Before locking the car he'd flick a switch under his dash so if the trailer plugs where disconnected his car alarm would sound until they where connected again. I think the alarm had a timer of 10mins or so as well. Had something to do with breaking the circuit or something. Again not foolproof as if it was the last thing they did they could have it on their car and driving off before anyone's suspicions where aroused i guess. But i would say a shock to those attempting the theft. He used to say it's good enough to deter the honest thieves!
Daz.