I had the same problem on my '65 c10... and at various times, these problems came up which I had to fix:
headlight switch
grounding issues at the rear lights
a short(broken wire inside the insulation) in various places in the wires... in my case, the wires toward the rear of the truck... and later I found a melted wire behind the fuse panel on the inside firewall.
a bad glass tube fuse(the original old style) which sometimes allows current across... mine was not black, but broken.... I replaced it and everything was fine.
Eventually, I did have to replace my switch even after I took it apart trying to clean it up... something was just worn out in there.
Anyway, some crazy things for you to check.
-W
