the problem here is that most people are going to recommend things that most other people that are on these forums already know, then some shady character will come on here as a guest and read this thread and see some new ways that people have isntalled kill switches on there cars and use that info for less then legitimate purposes
the best way to fit a kill switch is to interupt a primary circuit in the car that controls one the main fuctions that are required for car to run/drive. things like fuel pump, ignition, starter etc
best to figure out where to do it yourself and then not tell anybody how and where you have fitted your kill switch, especially not in an open forum.
get hold of a wiring diagram for your car and you will soon see something that will work, check out this site if you don't have a gregories car manual
Rob's Creative Calibrations , there are wiring diagrams under the "info" section