70 C-10 Wiring Problem

[Longhorn Man]

I disagree with everything said.
Headlight switch has no control over the turn signal indicators.
Bad ground, there would be fuel guage complaints too (I am assuming it works) and possibly eng temp guage complaints too.

Remove the insturment cluster, twist and remove all the light bulb sockets. There will be at least one hole with no light, mark it if you have to, or take digi-photos for reference. (a drawing would work too)
Replace all the bulbs while you have them out, it's only a few bucks.
Inspect the printed circuit. Where the bulb holders contact the circuit, look for dirt/corrosion/burn marks. Use a pencil eraser to clean them if anything is found. Do it even if you don't see anything.
While replacing the bulbs, you'll see on the holder, where the power comes in, the little metal ears sticking out. Clean them with some fine sand paper... the wife's fingernail file will work in a pinch, but something more fine than that if you can. You can also pull them away fromt he plastic body a little too. not much, just a tad.
Put it back together. Adding a ground strap that is probably missing is a very good idea, and I highly recomend it, (I still say that's not the issue, but if it is missing, it may well become the issue) I just use a section of wire. It doesn't need to be thick, speaker wire is plenty. Put eyelets on either end. Attach one to a clean spot on the dash, and the other gets bolted to the steel part of the cluster.