Get a wiring diagram and pin assignment list for the EEC-IV 60 pin connector (Is this something you have access to, Ken00?). Locate the STI, STO, and SIG RTN leads that would go to the self-test connector. Then, either trace them out to the fender under the hood and hook the connector up, or cut these wires and wire the self-test connector in somewhere else (under the dash seems like a good possibility).
Like I said before, the self-tests seem to be the best place to start. If it were my truck, I'd get the self-tests working before chasing down other paths.