Try this link to check your inside wiring harness. compare the idiot light guage panel connection to what you have in your truck. I just went through this...my truck had a 7 guage panel and nothing worked right, turn signals in dash acted weird and fuel guage didn't work. Turned out the truck did not originally have gauges. So I re-pinned the guage panel connection and now it works.
http://67-72chevytrucks.com/vboard/s...42#post5718242