i have an 86 iroc and it still has a 335 in it, obviousely 305 with a "LIL" work. but i installed headers a few years ago and shortly after that i noticed my temp gauge doesnt work. or my fan. the temp sender on the side of the block had no plug in it. i absolutely can not find the wire anywhere in the engine bay. there should be a wire going from the sender to a black box on the fire wall(forget what the box is right now its to early in the morning) anyone know what box it is and where its at. there is no loose wire hanging or anything that i can see burned off or ripped out....lol...my car is carbed, original dont know if that means any difference in the wireing. anyone who can help is a life saver. noticed the gauge didnt work a while ago, no big deal gauges go bad but one day in traffic, boiled over the overflow......figured out the fan doesnt work either.

 

Sounds like the fan switch wire has been cut real close to the harness somewhere along the line. I'd suggest taking the time to explore the passenger topside and underside of the car's wiring real closely. You're looking for one single wire that's been cut or has broken.

As far as the (temp) gauge goes, have you first tried replacing the temp sensor? Or, testing the wiring for opens?