I had a similar problem with recently acquired '69 Sovereign
. Got one weak tail light signal to one of Andy's LED lights (which was a replacement for the rusted out original cans). Presumably bad ground. So with a new trailerside connector wired to match the umbilical cord, ran a four wire trailer cable down the middle of the interior floor, out the rear window and thru the back up light holes with a ground wire screwed into the housing. Now have running and brake/turn lamps working - so ready to bring home end of this week. (Obviously no back up, charge or brake connection at the moment.)
Would appreciate any thoughts on how to check original wires from front to back. Or good way to run new wiring to back of unit (preferably without doing too much to the interior panels).
Thanks in advance,