You gave us the year and model which is a good start, but engine, tranny are a major help as well. Put a test light on the small terminal on the starter solenoid and see if it lights when the key is turned to the start position. If it does, starter. If it doesn't the next thing to check is the neutral safety switch located on the top of the steering column just before the column goes through the firewall, assuming this is an automatic. If its a stick its on the clutch pedal linkage. Try holding the key in the start position and with your foot firmly on the brake pedal slowly move the shifter from park to reverse to neutral to drive if auto. If it engages the starter doing this the switch needs to be adjusted, the mounting tabs on the switch are slotted. With out looking at the plug to jar my old memory, I cant tell you which two wires go to the starter from the ignition switch. get a wiring diagram, or perhaps someone will jump in and you can "jump" the two wires and see if it starts then. If it does then it is the switch, or again it could be out of adjustment, it too uses slotted mounting tabs.
Whitelightning
Sorry about not giving engine and tranny info. It's a 7.4L with a 3-speed automatic.
It appears the neutral switch is a plastic housing (that has some adjustment with plastic tabs. Tried moving it back and forth with the key in the start position; no luck. There are two connectors on the housing. One connector has a black/w white stripe and orange/w black stripe wires. The other has a green and a blue wire. Neither matches the wiring diagram I have (from a Chiltons GM Full Size truck repair manual 1988-93. The electrical system wiring diagram shows only a yellow and purple to the neutral switch.
The starter has one of the shields that makes it almost impossible to get to the solenoid with the truck on the ground.
Holding the key in the start position and slowly moving the shifter from park to reverse didn't get me anything either. Before I attack the starter I thought I'd check out the neutral switch. If someone can tell me which two leads need to be jumped, I'll remove the connect and try that. One connector comes off easily ( the one with the black/w white stripe and orange/w black stripe wires ). I haven't figured out how to get the other one off yet. It isn't apparent where the hold-down tang is and it doesn't want to come off. It's impossible to see what's holding it in place so I can only guess there has to be a tang there somewhere.
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