Trailer lights - not good

[RRConboy]

My truck has factory tow package, so the truck wiring to the fuse box should be good. Socket is standard white, brown, yellow blue. (Ground, tail light, left turn, right turn). Problem: Left lights are dim. Right lights are bright - everything works - tail, stop, turn, flasher. Left lights are weak (which I think indicates a weak ground). Tail, turn, flasher work, but dim. BUT, when I hit the brakes the whole left unit goes off! Right unit still going. I bought a new set. Plug in the new set (white, brown, brown, yellow, blue) to the truck socket. Make sure the mounted left unit is grounded to the frame. Ground the new set to same spot as the old set - frame tongue. Both sides getting the same grounding, it seems to me. Run new brown-yellow to left unit. Run new brown-blue to right unit. (Just running wires on the outside, on the ground, for testing). Getting same results - weak left lights, all work, but brake shuts the whole left unit down. Right unit still going strong. I am stumped.