While browsing my collection of tech notes, I found this one. It uses the safety lockouts to prevent the car from being started while in gear.
Tools needed:
#3 Phillips screwdriver
Small jeweler’s screwdriver
Heat source to shrink the heat shrink tubing.
Soldering gun
Rosin core electrical solder.
Parts needed:
AutoZone Ford electrical connector pin kit
Push button starter switch
18 gauge stranded automotive wire
3/16” heat shrink tubing 6” or longer
1/4 “ heat shrink tubing 6” or longer
3/8” heat shrink tubing 6” or longer
2 each 18 gauge ring terminals
12 each small tie wraps
Make sure the ignition switch is off when you do this. I recommend that you disconnect the ground cable from the battery if you don’t mind losing your radio and computer settings. The computer will relearn the settings after about 15-20 minutes of highway driving.
Remove the plastic cover from around the steering wheel: it is secured in place with 3 Philips head screws.
Remove the ignition connector from the ignition switch. From the front side, use a jeweler’s screwdriver to lift up the plastic finger that holds the white/pink wire in place. Pull the wire out from the rear. Crimp the mating pin from the AutoZone Ford electrical connector pin kit onto the 18 gauge pigtail. What you are doing is adding an extension onto the white/pink wire, so you use a male pin to plug into the female pin that was in the ignition switch connector. Doing it this way will allow a neat and easy way to return to the stock configuration. Put a 1 1/2” long section of heat shrink tubing on the pigtail and then join the connectors. Slide the heat shrink tubing over the exposed contacts and heat until the tubing is uniformly shrunk all around. This will insulate the contact pins and keep the pins from pulling apart. This is one of the wires that will go to your pushbutton, so give some thought to how long you want it to be.
From the front side, use a jeweler’s screwdriver to lift up the plastic finger that holds the red/ lt green wire in place. Pull the wire out from the rear. Scrape a 1” section of the insulation off the wire. Strip 1” off the 18 gauge wire you are using to connect to the push button switch. Wrap in around the bare place on the red/lt green wire and solder it. Cut another 1 1/2” piece of heat shrink and slide it over the bare solder joint. Apply heat until the tubing is uniformly shrunk all around. Slide the red/lt green wire back into the plastic connector shell and make sure it locks in place. This wire provides power to the computer, fuel pump and ignition, so you don’t want it to come loose from the connector shell. Route the two wires down the steering column so that they do not snag anything and tie wrap them in place. Replace the plastic ignition switch cover
Find a suitable place for your push button switch and drill the mounting hole. Crimp a ring terminal on the end of each one of the pigtail wires you ran from the ignition switch. Cut a 1” piece of the 3/”8 heat shrink tubing and slide it on each of the pigtails. Connect the pigtails to the screw terminals on the push button switch, then slide the heat shrink tubing over the exposed screws. Apply heat until the tubing is uniformly shrunk all around. This will protect the wiring from short circuits since the power side of the push button is hot anytime the ignition switch is in the Run position.
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring
Mustang FAQ - Engine Information
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Computer, actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Fuse panel layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif
Vacuum routing
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg