I saved this from a thread a while back, but I do not remember who's posted this, so sorry for no credit. BTW I have not tried this yet myself. Here goes:
A Fuel Cut-Off switch is one way to stop your car from being started and sense our Mustangs have electronic fuel pumps it is very easy & inexpensive to install a Fuel Cut-Off Switch.
All you need is a simple ON/OFF switch and some wire and an hour or two and you will have installed a Fuel Cut-Off Switch and if it is set in the off position you will not be able to start the engine.
The fuel pump has 2 wires going to it, all you have to do is splice into one of the wires; it does not matter which one, and insert an ON/OFF switch. There are many places that you can splice into these wires and install a ON/OFF switch, that is totally up to you. I spliced into the wires at the fuel pump on top of the gas tank to make it harder to find the splice. You can find the wire and switches you will need at Radio Shack and it should not cost more then $20.00, depending what kind of switch you buy. Hide the ON/OFF switch in a place that only you know about and is not easily seen. If you have any questions PM me and I will be happy to help you out. Below is a diagram I made to show you how simple the wiring is.
Where can I find the wire to splice, what color should it be, and what type of wire must I buy from radio shack to attatch to the wire i cut and to the switch ?
On 86- 91 cars the fuel pump relay is under the seat, 92 and later cars, the fuel pump relay is under the Mass air duct on the passenger fender well.
Note that the wire colors changed in 1991, so there are two different descriptions.
86-90 model cars:
Cut the red/black wire going to the fuel pump relay. Splice a 16 or 18 gauge wire into the wire and connect it to one side of the switch. Connect the other side of the switch to the other end of the red/black wire going to the fuel pump relay. When you are done the switch should be in the middle of the spliced red/black wire that goes to and comes from the fuel pump relay. Using the control side of the circuit allows you to use light gauge wire and light duty switches. There is less than 1 amp going through the circuit, so you don’t have to worry about voltage drop across the wiring depriving the fuel pump of electrical power.
See http://www.autozone.com/servlet/UiB..._us/0900823d/80/16/71/3c/0900823d8016713c.jsp for 79-88 model wiring diagrams.
91-93 model cars:
Cut the Lt Blue/Orange wire going to the fuel pump relay. Splice a 16 or 18 gauge wire into the wire and connect it to one side of the switch. Connect the other side of the switch to the other end of the Lt Blue/Orange wire going to the fuel pump relay. When you are done the switch should be in the middle of the spliced Lt Blue/Orange wire that goes to and comes from the fuel pump relay. Using the control side of the circuit allows you to use light gauge wire and light duty switches. There is less than 1 amp going through the circuit, so you don’t have to worry about voltage drop across the wiring depriving the fuel pump of electrical power.
See http://www.autozone.com/servlet/UiB..._us/0900823d/80/19/59/5a/0900823d8019595a.jsp for 89-93 model wiring diagrams.