I was just working on my 50 Chevy pos,the lights have the same problem as yours,ain't that something.Not a grounding problem.I jumped the dimmer, no change,ran a 12 gauge jumper directly from the battery to the lights, still dim.Disconnect one headlight, the other gets bright, hook the wire back up,gets dim again.Reversed the procedure with the other headligh, same results.So for kicks I swapped the hi and low beam wires on one headlight,now both are normal brightness on hi and low beam.
I still question the terminations on your headlight bulbs. If you have a spare lamp, put a rag on it, smash the front with a hammer.Look to see with terminal is the ground,it'll be common to both filiments.Despite what may be supposedly correct, mine had the wrong hook up.
