You at least looked for schematics. There is a single 4 wire cable from the neutral switch that is routed from the left hand side of the transmission over the bell housing to the right hand side then up to two two male-female connectors at the rear center of the engine compartment. They then go to another dual two pin M-F cable routed thru the firewall. One pair is for the neutral safety switch circuit (both wires are red-blue tracer), the other pair for the back up lights (both are black-red tracer).
The neutral safety circuit gets it power from the back of the ignition switch, the large rubber connector, and feeds the completed circuit to a multi pin connector on the firewall to the starter solenoid. All wires in the circuit are red-blue.
The back-up lights get power from a triple female connector behind the instrument cluster, and feeds switched power to the rear of the car thru the under dash harness and the tail light harness. All wires in the circuit are black-red.
I recommend you get a set of Mustang Assembly Manuals for your year car.
Jim