Go inside the car, take apart the steering column covers to find the wiring that comes down from the key cylinder, and find the wire that is hot when the ignition is on AND continue to stay hot while the starter is cranking. It must be hot for both scenarios, otherwise you have the wrong wire.
Then once you have the right wire, cut it, and make sure nothing happens when you turn the key to ignition, and then wire your switch from there. On my chevy, I screwed my switch to the seat bracket but at the very top, so the seat covers it for the most part. On my mustang, I took the seat cover off, cut out some of the foam, added some epoxy and glued it to the foam support. No one will ever find my switch!!!! muahahahahahha
As far as hiding the switch, it's all about how far you want to go. If the car won't start, and it doesn't have an alarm, most thieves will just give up and move onto another car. I chose the fuel pump because where I spliced, they will never find, nor will they have the intelligence/patience to figure out a way to bypass it. A manual kill switch is so simple yet so effective. All the fancy electronic passive systems don't stand a chance to an active system like a kill switch.
Scott