I just picked up a 1987 s-10 Blazer I thought the fuel gauge didn't work at all, but as I was checking fuses and doing a few other small things I realized that the gauge does work, but not when the headlights are on.
I don't know much about cars, but this seems like a weird thing to me. Any suggestions would be greatly appreciated.
-Kelly

 

Sounds like there are some wires crossed somewhere in the dash. It could be a problem with the cluster itself.