Had a Very similar issue with my 66'. After replacing turn signal switch, headlight switch, AND dimmer switch, I turned my attention to the rear of the truck. Turned out the tail light housing did not have a good ground to the bed. I ran a separate ground wire from housing to the frame and BINGO! Lights have never been brighter and everything (tail lights, running lights, turn signals, etc) works perfectly. I have had other classic cars with electrical issues and bad grounds were the root cause. Kicked myself in the ass for not checking the grounds on my 66' FIRST before replacing parts
