When you notice your head, tail and dash lights are often dim, sometimes even flicker on a rough road, check your cab to frame ground cable
Because the 1967-1972 cab and radiator supports are seperated from the frame by rubber mounts. GM used a small mount woven wire ground strap that by-passes one cab mount. This insures electrical flow even if the cab mount bolts become rusted and electrical current can not flow properly.
You must be under the cab to see this by-pass cable. Yes, GM planned for the trucks later years when rusty mounting hardware caused the lights to dim