Quick work around for that Park issue. Either connect or disconnect that circuit and see if anything changes.
I'll check my '89 factory manuals, but I'm pretty sure it would just be an off/on switch or ground/not grounded signal. The mid to late 80's computers were pretty simple like that. So, if it is currently not grounded, ground it, drive it, and see if anything changes. Etc. At least, that's what I'd do. Gimme an hour or two and I'll dig up the manuals for the wiring diagram.
