Most manual trans. vehicles have a switch on the clutch pedal so you can't start the engine with the clutch engaged, automatics also have the wiring there, just no switch or pedal, just wire in a hidden on/off switch It took 30 minutes to install one on my truck.
A mechanic friend of mine knew I had a switch hid and still couldn't find it within a half hour. Add a steering wheel club for a visual deterrent so they don't bust the side window, and it will stop 99%+ of the low lifes.