factory tach only works when dash lights are out

Sounds like a bad ground. The dash lights or dimmer swich is grounded at the same place, so when the lights are on, the tach cannot ground.

That sucks, either you can't see the working tach, or you have to look at a tach that isn't working.

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.