That sounds like a grounding problem to me. On the '66, at the rear of the dash, there is a metal grounding bar that attaches to the steering column. I forgot to tighten that up securely and had very strange dash light operation. Was a really quick and easy fix. Not saying that's your problem, but it may be worth a look. Also, have you pulled the fuse box and inspected the terminal ends and connectors? Mine were RUSTY. Took FOREVER to clean and deoxidize, but well worth it, saved a bundle of cash, and after all said, was gratified to still retain the original wires (NICE) and fusebox, with fresh loom tape. I also installed 4 new engine to frame/firewall/body ground straps. All is swell in electric land. And as Pontiac Jim said, check those old wires laying on top of the engine going to the alternator which have heated and cooled 100,000 times. They get mighty brittle after 45 or so years. May the Pontiac gods smile upon you!