The gps never worked on that truck. The controller was smashed apart when I got it. I had a manual setup that worked for a while until my knee hit the switch while getting out one day. I'm pretty sure that fried what was left of them.
It would be easier to rewire the system, like I'm doing on my '90 atm. I don't have good wiring diagrams either, but a good meter with continuity test helps alot. It may be dangerous to assume that what he's looking at will match a factory diagram, as it looked like it might have had a CB, beacons, etc. at one time. Not to mention the liftgate, which has an ignition relay signal wire as I recall. Also, International makes a conversion kit to switch over to the 7.3-type system, it includes controller, harness and gps.
Once again, tell him I truly am sorry. That truck was crash course IDI 101 for me.
Just found this if it helps:
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But as I said, most of that wasn't there anymore.

I have some engine pics of it still, but none of the fender with all the business on it.