[TheOmniscientRT]

a relay would work best, it will act a a fuse for one thing, a switch could add resistance make the part not function as well, a relay wouldnt, you could hide the rlay close to the part(ecu, pump, ect) and stick the switch out in the open. A car theif is first gonna look under the dash to find extra wires to see if there is a security system and rip them out or bypass them. everything a car thief needs to steal a car is right at the steering colum, so any securtiy device you put in needs to be some were they are not going to look.
If you really want to be sneaky. Use a relay on the starter wireunder the hood and run the wires to a momentary switch. That way the starter wire can only be powered when the button is pressed, so you have to press the button or when you turn the key it does nothing. this will alos keep some1 from hotwiring because putting power on the starter wire will not send power to the starter thanks to the relay. You could then hide the memontary switch, say under the ashtray, that way all you have to do is push down on the ashtray a little bit and turn the key and she fires up. not pushing on the ashtray would keep it from starting. its less obvious than pushing a button.