The clutch switch is your best bet, if you're looking for a starter circuit interupter.
It's a dual circuit switch. One part interupts the starter circuit, and the other part reports to the computer. The part that reports to the computer is also a part of the 5-speed's neutral switch circuit.
If you want to scan for the Key On Engine Running codes, you'll need to jump that circuit. The computer either needs to think that the
tranny is in neutral, or that the clutch pedal is being held down, to throw the Engine Running codes.
I suspect that the clutch switch uses 4 wires, 2 for the computer, and 2 for the starter. You might have to wait till you're ready to start the car, and see which 2 wires (when connected) allow the starter to crank - just be careful not to mix the computer circuit with the starter circuit - study the switch, to determine which 2 wires go to each circuit.
__________________
1992 Deep Emerald Green Mustang GT Hatch, 5 speed, 2.73s, Mac fenderwell cold air intake, '93 Cobra MAF, Mac 70mm throttle body & spacer, Explorer intakes, Mac unequal headers & offroad pipe, Magnaflow catback, Walbro 110lph fuel pump, Jet adjustable FPR, MSD Blaster coil, Accel 8mm wires, FRPP aluminum quadrant, UPR firewall adjuster, stock cable, Granatelli upper control arms.
Waiting to go on: Granatelli lower control arms, and GT40 Iron heads!
Manual to C-4 Swap - Neutral Safety Switch Question 
