Have installed hidden switches in all my cars, but they all inhibit the starting circuit:Works just like the switch that you have on every automatic transmission that prevents starting except when in P or N. Switch on the fuel pump isn't so effective, because on the W113, when the tank is full, it'll still run. You must also park the car on gravel, dirt or grass, so that dollies and jacks are difficult to use. Nice thing about the switch: You can flip it while you drive, it just inhibits starting the engine, not running it.
'69 280SL,Signal Red,Automatic,retired engineer, West-Seattle,WA