Find your reason to install a kill switch

What is a kill switch?
Kill switch is a type of antitheft device; in essence, it is a kind of engine disabler or starter interrupter. Kill switch is not used to stop a running engine. It is employed while the engine is still off with the aim to prevent it from starting. Once activated, it blocks the circuit an engine needs to start. Its installation is pretty straight forward and the steps are easy to follow. A wire is required to be cut to let the kill switch spliced in.
To reenable the engine, you flip a switch or push a button. Sometimes a kill switch is wired to connect to the windshield wipers or turn signal, which then must be activated before the vehicle can start.

Why install a kill switch?
The most common reason to install a kill switch in a vehicle is to prevent amateur and low-level thieves from stealing your car or truck. A kill switch will also slow down a professional thief (discussed more in the next section).
Other possible reasons would be to keep unauthorized people (senile relatives, employees, teenagers, etc.) who might nonetheless have access to your keys from driving a vehicle.
A much less common usage would be to kill a vehicle that is already running, which I got a request for in 2010 when Toyota vehicles were accused of running full speed ahead despite the driver's intentions.
Unlike car alarms and vehicle tracking systems, kill switches do not draw power from your battery, so you don't have to worry about them draining a weak battery if you don't drive your vehicle for all while (like if you are on vacation).
I protect my own vehicle with an EZ KillSwitch.

Why not install a kill switch?
3 weaknesses of typically-installed kill switches make them easy for professional thieves to bypass, because
1) Professional thieves know most owners hide the kill switch in a location that is convenient for the driver.
2) Once found, a kill switch normally just has to be toggled (flipped or pushed) on or off to allow the engine to start and the thief to drive away.
3) And most kill switches simply make a connection under the dashboard, which the thief can find and jump even he doesn't find the actual toggle switch or button.
Knowing those weaknesses in advance, you can modify your installation to make it more difficult for a thief to take your vehicle.
Many vehicle owners don't want to cut wires on their vehicles. If you want a kill switch but don't want to cut and solder (or otherwise reconnect) the wires yourself, either get an EZ KillSwitch that almost anyone can install in 5 minutes without any wire-cutting or call a car alarm installation shop and ask them how much they would charge you (expect them to charge about $200).

How to Install a Kill Switch
(This is part of our "Kill Switch Guide". You can also read about what a kill switch is, and and why you should or should not install a kill switch.)
I'm going to present 3 different options. Pick the 1 that suits your skill level, budget & theft-risk situation the most.
First, the 2 easy ways:
Option 1, Do-it-yourself EZ (no cutting necessary)
Step 1. Order an EZ KillSwitch (available for many models 2001 and after). Picture instructions included. Guaranteed against theft ($1500) in many states.
Step 2. Following the pictures, replace the specified relay under your car's hood with the EZ KillSwitch relay. You are done: installation complete. Without pressing the button on your key fob, a thief will not be able to start or hotwire your vehicle. Not even if he smashes your steering column. He can search in the cab and under the dashboard of your vehicle all he wants -- there will be nothing there for him to find.
Option 2, Easy Outsource (pay a professional to cut a circuit or two)
Step 1. Call around for installation prices from car alarm shops that are willing to install a kill switch & make an appointment.
Step 2. Take the vehicle over, pay the man and wait about 30 minutes.
If you are more comfortable working on your own vehicle, you can go with the do-it-yourself, harder but cheaper way.
Option 3, DIY Cut a Circuit or 2
(Disclaimer: This is a general guide, please consult an installation professional for your own vehicle.)
Step 1. Decide which wire or wires you are going to interrupt. If you don't drive a high-risk vehicle and/or live in a high theft location, interrupting one circuit in the cab of your vehicle should be fine. If you are in a higher risk category, I recommend 2 kill switches (ideally on 2 different circuits), at least one of which is in an inconvenient location for the driver, and at least one of which makes its connection in the engine compartment under the hood of your vehicle (instead of just under the dashboard).
For help deciding which circuits to cut, call or stop by your car dealership. The most common circuits to interrupt would be starter & ignition. Professional thieves, however, are aware of that so you might want to interrupt some other circuit.
Step 2. Decide where you are going to hide and mount your kill switch, and whether you going to have a push-button or flip kill switch. Or one wired into your fog lights, defrost button, turn signal, whatever (those are beyond the scope of this article). You also need to decide how to hide the wiring so it's as hard as possible to find and ideally looks factory installed. If you're connecting under the hood (recommended), whenever you get a chance, run the wires through the factory installed wire heirloom.
Step 3. Purchase the supplies you need from a hardware and/or automotive store.
Step 4. Start cutting & splicing, soldering and/or crimp-connecting. Remember, the more hidden & factory-installed-looking you make your installation, the tougher it will be for a thief to defeat. And you'll do better using a less-common circuit.