Originally Posted by sxng9

the easy way to know for sure that you have tapped the right wire is that when you cut it and then try to start the car, it should stall. If it stalls, then you cut the right wire, if not then you need to re-check all the wires & look for the right one.

once you found the right wire and the car stalls when you try to turn the car on, then you apply the switch to both ends of the wires. it doesn't get any easier than this and you don't need to ground anything, assuming that it's a simple fuel cut-off switch.

if it's a complex one like mines, then you'll need a relay, a constant 12v power source & ground and also on my car, as an added security feature, I carry an RFID card in my pocket, so that if a theif happen to drive my car off when i'm outside the car and the car gets outta range from the RFID card, the car will automatically stall & alarm will start blaring off.