[VetteVet]

Do you have power to all the things that are keyed on hot I.E. wipers, radio, turn signals,
heater or AC etc.
If so then the key switch power wire (large red) has power and the key switch is good. The purple starter wire and the pink ignition wire have to be jumped to get the engine to start and run, however the purple wire must be disconnected when the engine starts for obvious reasons. The key switch might be bad for the starting circuit, Large red to purple, but could be ok for everything else. Checking for 12 volts on the key switch and the purple wire when the key is turned to start would show this, as would checking for 12 volts on the purple wire at the solenoid.

Jumping the neutral start switch is a good check for the switch but unless you know you have power on the key side of the purple wire, it won't tell you much.

If you know you have power on the large wire to the key switch and all the other stuff in the cab works Then the purple wire is the problem. it starts at the key switch and breaks at two places, the NSS and the firewall connector. You have already jumped the NSS so the firewall connector is the next suspect. There is also the possibility that the purple wire is cut or burned in two, at some point.