The car is down for winter maintenance items right now but it runs 14.5V at idle. ERA runs grounds from each tailight frames to a big bolt for the bumper brackets and I install serrated washers at all grounds. All are tight and I have multiple grounds installed from the drivetrain to the frame. Everything functions properly and is bright (for filiment bulbs). If I had ground problems I think it would show up in the blinker and brake light circuit also, which worked fine with the LED bulbs.
I mean the LED bulbs are working fine in my GTX with a 47 year old wiring harness, taillights and Mopars known reputation for electrical issues. A large number of these things burned up by the mid-70s due to electrical problems. If it was simply a bad ground or resistance issue that was the problem I can't explain how they could possibly function in my old Plymouth.
I appreciate the pointers. When I get the car out again and it warms up I may get out my meters and do some more checks on the tailight circuit. But I kind of wore myself out earlier, trying to fix something that wasn't obviously broken. I'm curious to see how Large Arbor does with them.