[67ss&99ss]

Have you tried the resistor that is on top of the heater box inside the cab?

I too have a problem with the lights on my truck. They all work but when the truck is running they dim and brighten back and forth and I noticed that the amp gauge oscillates at the same frequency as the lights. I have a new ground strap running from the engine to the cab but all of the others are older ones. The ground wire for the battery is attached to the passenger side cylinder head and I made sure this was a clean connection when I put the engine in. Should I replace all the other ground straps with new ones or is here something else causing an issue? The truck has the original harness, but it has never been cut/spliced. I replaced all the bulbs and put di-electric grease in each socket so they are getting a good connection that is water tight.