boogered wiring is a PITA !!!
IMO, the first thing to address is look at the wiring diagrams and determine that EVERY ground that should go to the chassis AND engine is good (i.e. clean surfaces, connectors solid. I've been through more trackkicks than samis, and the track/kicks have probably a dozen or so different grounding spots (i.e. chassis ground versus engine ground versus battery negative).
Next, I'd unplug everything that isn't essential to the running of the engine. Unplug all lights (dash lights, dome light, head and indicator lights, radio(s), etc). I'd also disable (temporarily) "safety" switches (e.g. the brake/park disable circuit).
Sometimes it's also necessary to disconnect the alternator as well (some people just love to tie power directly from the alternator).
From here, get the engine starting, idling, running properly.
Then put back in the "safety" stuff, and verify it's adjusted and working properly
Then start with each lighting circuit one by one.
Eventually you'll trace back and find that wad of black tape (or worse scotch tape) that someone "tapped" ...
Good luck and from those of us that have dealt with this crap before, we feel your pain/frustration.