The best way to keep someone from stealing your vehicle is to have a normally closed solenoid valve installed at the tank that shuts off the fuel flow and vents the downstream line to air if the correct procedure is not followed, the solenoid can be controlled by a relay that requires another vehicle system to be switched on before starting use a system that normally would not be thought of. If you use a DPDT relay it can be made into a latching relay which will remain on after the vehicle is started until the ign. is switched off once again the secret system could be just about any electrical system and can be used only as momentary contact or left on.
Works on all vehicles even older carbureted with or without fuel pumps in the fuel tank. Even if the vehicle starts and runs it will only do so momentarily. Works especially well for vehicles owned by persons who might be inebriated. Will now work on vehicles you want to prestart before getting in.