Hi there,
Your options depend greatly on whether you have a gasoline or diesel engine. If you do have a gasoline engine the switch on the fuel pump relay is a good option, as long as it's well hidden. If you have a diesel engine, the easiest place to put a switch is in the starter solenoid circuit. Here in the Washington/Ore area I feel pretty safe with my Mercedes because the keys are a little different than most rigs.
Good luck with it!

Richard Wooldridge
'82 300D/4.3L V6
Etc...