Is this an automatic transmission.
If it is an automatic transmission the neutral safety switch could be bad. If it was mine I would get an electrical schematic, and start checking out the wire that takes power to the starter to trigger it into action. Odds are your problem is a wire that is broken, or a bad connection. If this is the case you will have to trace or bypass the the wire from the ignition switch that triggers the starter.
do you have digram of the wires from the switch to the starter
I will see if I have one I can post.
I have it, and I also found there is an additional relay that might be the problem. I cant post some material because of the laws, but I still found a way to help. Check out this link HERE, it should help get your starter working again.