Here's the neutral start switch on top of the blue steering column. The purple wires come from the key switch and go to the starter solenoid. If you pull the plug off the NSS and jump the two purple wires then you are by passing the neutral start switch. The truck will start in gear so be careful.
This is the starter wiring circuit.
To isolate the starter from the purple wire, I take the purple wire off the S terminal and jump across the solenoid between the battery cable terminal and the S terminal. I use an old screwdriver or wrench because it will arc. leave the nut on the S terminal to avoid messing up the threads.
If the starter turns then the problem is in the key switch to the S terminal wiring. If it doesn't then it's in the starter. Make sure the truck is in Park so it won't move and leave the key off so it won't start. The plate in this picture is for a remote solenoid so disregard it.
Here's abetter picture.
