Here is an option:
Take a look at the fuel pump inertia switch, located on the driver's side rear in the trunk. make a note of the wire colors at this connector. one wire is brown, the other wire is green with a yellow stripe. you will be splicing into this green/yellow wire as it snakes through the passenger compartment.
Pop off the plastic trim along the bottom of the door opening, on the driver's side. the wiring harness runs underneath the carpet. pull up the harness and tear off the plastic wrap to expose the wires.
Now, look for the fuel pump wire (green with yellow stripe). MAKE SURE YOU FIND THE RIGHT ONE! there is another wire that looks similar (olive green w/yellow stripe). you don't want that one.
For safety, you can verify the right wire with a meter. here's how:
Stick a safety pin in the green/yellow wire that you found in step
The pin must be deep enough to pass through the insulation.
Connect a piece of wire to the pin, long enough to reach the trunk.
Go to the trunk, and remove the inertia switch connector.
Set your meter to read RESISTANCE (ohms). connect one lead to the inertia switch connector's green/yellow wire.
Connect other meter lead to the wire that you connected to the safety pin.
If the meter reads 0 ohms every time you connect the safety pin wire to the meter lead, then you got the right wire. if the meter reads anything other than 0 ohms, you have the WRONG lead.
Once you've found the correct green/yellow wire, it's just a matter of splicing in a switch to connect/disconnect. for safety reasons (or if you want to use a tiny hidden switch), you may want to use a relay, which is what I did. I used a 30A 12VDC SPST relay (it's much bigger than needed, but that's ok). to use a relay, follow this:
Cut the green/yellow wire
Wire both ends of the green/yellow wire to the 12V in and 12V out leads on the relay (it doesn't matter which lead goes where, as long as it's the 12V in/out connectors).
Connect about a 5ft piece of wire (2 leads, like speaker wire) to the coil side of the relay, noting which wire is connected to the relay's ground side.
Snake this wire to your chosen hidden spot for the switch (I chose a small hidden spot near my ashtray).
Connect the ground wire to a nearby bolt, making sure the bolt contacts the car body
Connect the other wire to one connector on your switch.
Connect the other switch connector with a piece of wire, and route that wire to a 12V ignition source (any wire that produces 12V when the key is in the ON position will work (use the meter to verify).
ALL DONE!
When the switch is opened, the car will try to start, but won't run. flick the switch on, and you're off and running. email me if you have questions. these instructions should work on the '94-'95 mustangs (and maybe newer ones). now, this isn't the only way to do this. some people like to splice the wire at the inertia switch at the trunk, and the route the wires into the passenger compartment. while this would work too, the disadvantage is that a SMART thief will look there FIRST! if a thief finds the relay (or the splice), it's a simple matter of running a jumper wire to bypass all your hard work. using my way, the relay is hidden underneath the carpet by the driver's seat, making it very hard for the thief to find it. of course, he can run a jumper from the battery to the inertia switch in the trunk, but he'd have a wire whipping around while driving your car
