1967 F-100; I replaced a dim headlight then successfully tested it with the truck off. When the truck was running, both headlights dimmed every 10 seconds. I went back and reconnected both headlamps inside their assembly and casings with another successful test. But, when the truck runs (lights on), the parking lamps and rear lights are off (brake lights work); they blink on every 10 seconds. Short in the light switch or loose grounding wire or fuse/battery problem?