You should just duct tape a flashlight to the steering column at night and kill your dash lights. Its a POS anyway, right?
Only kidding, look and see if you cant tell where the two circuits meet. These diagrams from Tmoss are pretty easy to read, its 88-91 but yours should be the same or similar. If I wanted to be cheap, I would hook a new signal wire to my tach from the neg. side of the coil, and a fresh ground. You know it works, just hook it up right.
I pulled that diagram, I dont have the right one. Sorry. You can look at the local library for a free wiring diagram, if you dont have the cash to buy a book.