There are several possibilities, bad wiring being one. A faulty ignition switch being another. The ignition switch isn't physically attached to the key tumbler. There is a plunger rod that runs between the key tumbler and the IGN switch. The IGN switch is attached to the top of the steering column behind the dash. When you turn the key, the plunger rod acts remotely to cycle the switch to and fro between the various detents or positions. First thing I suggest is hit up your local parts store and buy a diagnostic starter switch, aka "Loser Switch". If you've never seen one, what they are is either a thumb button or a pistol grip with trigger, plus a two wire lead that has alligator clamps on the terminus end. They're inexpensive and super handy. What it will let you do is crank the starter while under the hood by yourself without even needing to touch the key switch. All you do is connect one gator clamp to the large solenoid terminal and the other gator clamp to the small solenoid terminal labeled 'S'. With your disconnect at the ready, try starter cranking first with the IGN switch OFF. Does the starter run on? If not, try it a couple more times just to be sure. If all is well, you can next rig a temporary jumper wire from the battery POS over to the coil POS terminal. This is going to completely bypass the IGN switch. Caution, don't leave this jumper wire connected until you are ready to attempt an engine start and don't leave it connected after you are done testing. This wire will be your kill switch if the engine does fire up and run. Yank the wire and the running engine will stop.
