Re: making a kill switch what wire?
Another one that my coworker did was the switch for his power window lock out. Had to have the lock out on before the fuel pump relay would come on (Toyota Four Runner). Another one I just thought of is if you have a two speed wiper motor use a 3 speed switch so that could be an ignition feed. I personally have liked using the fuel pump relay method (if you have electric pump) because as TR says, it will crank and run the battery down but won't start.
Grounding the negative side of the ignition coil is also very effective. You can come right off the tach feed and do it with minimal wiring also. Nickmil.