Below is an old write-up I put together in the past. Some of you have already seen it but I offer it here and now as contribution to this site.
Hey guys, here's a list of things I have wired on many many cars and is guaranteed to deter the average idiot thief:
1) Starter kill:
Pop open and take down the removable panel underneath the steering column. If you follow one of the harnesses down from the ignition switch it goes to a plug on the fuse block. Look for a medium sized (about 12-14 gauge) wire that is black with a white stripe. Cut that wire about 3 inches from where it joins the fuse block and put a momentary on push button switch in between it. The switch only has 2 leads so one side of the cut starter wire goes to one side and the other goes to the other cut side. You can then hide the switch underneath the dash somewhere where only you know to reach down to push it to start your car. -or- If you feel creative, you can fabricate some way to mount it under the carpet where your left hand or right foot can reach so it's hidden from plain view.
2) Fuel pump cut:
Locate the main relay underneath the dash above the driver's side kick panel. The relay is mounted on a bracket then directly to the metal there. You will want to cut the wire that's yellow with a black stripe. Some people have gone all the way back to the rear for this wire but that is unnecessary because that wire is the same wire that extends from the main relay. Once located and cut, do the same thing to it as with the starter interrupt except you will want to use a toggle switch instead of a momentary push button.
3) ECU disable:
In the underhood fuse block there is a series of wires that come off of the main engine wiring harness and plugs to it. Locate the wire that is yellow with a white stripe. That wire is the power feed to the ecu and with it cut, no amount of anything will start the car. The disabling principle is the same as the fuel pump cut. Again as with the fuel pump cut, a momentary push button switch will not work of course cuz you want to be able to turn the component of choice completely on or completely off, not just momentarily. Momentary switches are only used with starter kills and the next item on the list.
4) Clutch switch interrupt:
Remove the lower column panel. Undo the two 10mm nuts that hold the fuse block in place. Pull the fuse block out a few inches away from its mount point to free up some room (this will allow you to stick your left hand in to help your right hand as you handle the clutch switch wires) Next, locate the clutch switch at the top of the clutch pedal assembly. *note - there is another switch at the bottom of the pedal assembly on some vehicles. I believe it was either for cruise control models or hf models. Do not touch that switch, it does nothing to help disable your car. Once you've located the clutch switch, you will notice a small harness/shroud with 2 wires in it (a black and a blue I think) that extends from the underdash main harness and goes up to the clutch switch. Next, cut one of those wires (doesn't matter which one) at a good location that will allow you to grab it and work with it. Put a momentary switch in between the wire you cut.
Here is a picture of 2 relays that I wired for my horn to honk if I forgot to turn off my lights. If you want I can explain how that works too. Just let me know. Actually, you can even use relays to enable or disable any component you choose as well. All of the things covered above can be controlled with a relay. The toggle or momentary is just a simpler way of accomplishing the same task. However, with the relay you can for example enable the starter only when you have the emergency lights on or turn the headlights on first. Relays allow for neater tricks to be done but you pay for it with with a headache. (the size of that headache depends on how good your understanding of basic electricity is)
There's not much to write up. I bought the kit here. General installation instructions were included.
It was just a matter of soldering the included relay to the keypad, putting it in the car in a location of choice with double-sided tape, and supplying 12V+ (nonswitched) power and ground. And then of course interrupt the wire of your choice (ECU main power, fuel pump, starter relay etc.). I also used my dremel to open up a hole in the back of the ashtray, to route the wires through. I didn't think to grab pics of the process.
It's not available anymore though, according to the website.
That's the same keypad that I have. Cutting it out won't solve anything, unless they find the interrupted wires and manually put them back together. Use multiple relays and multiple kill switches in combination to avoid that.