Kill Switch

I have an 89 Century and would like to put an ignition kill switch in it. I called a few alarm shops but they all wanted $50 plus for the installation. I wanted to know if any of you have a done this before, and could tell me which wires I need to hook it up to. Most people just say
"Hook it up to your ignition wire, its red or yellow", but how can I be sure? I also know there other ways, like wiring it to your carb, EFI, or coil, but those seem to be a lot harder. I want the switch to be just a regular on/off switch mounted inside the car. My car is EFI, any help would be appreacated.

DO NOT TOUCH any YELLOW wires, these almost always lead to an airbag, and messing with them may inadvertantly deploy the airbag.

Edit: Had to correct my horrible spelling

You can also put the switch in the fuel pump circuit. This is easier if you are a novice. Get an in line fuse holder, some spade conectors, and your switch. You then install this in place of the fuel pump fuse. Not as effective against a professional theif, but a joyrider will not go looking to repair or defeat you car. Besides, a professional will know how to defeat most kill switches and alarms. For a pro you need a LowJack. I doubt a pro will be after a 89 Buick Century.