So my 280zx has a dim right headlamp. The right lamp tends to get really dim when hi beams are on. I been searching around and still can find a way to fix it. I tried replacing lamp, didnt work, then i checked the fuse and switched them around and still didnt work. When i was playing around with the fuses i took the fuse from the left lamp and placed it into the right lamp's fuse (leaving the left lamp's fuse empty) and turned the lights on and the lights didnt come on at all. Came to my thought that the right lamp is feeding off of the left lamp for power for some reason. I also checked the combo switch to look if its burnt but it didnt seem like there was any problems with it, i guess i will take it out and clean it anyways. I have no clue where the ground is located to see for corrosion or loose wire, where is the ground located? Is it possible though that the right lamp is feeding off of the left one?

 

I noticed that my Right low beam is weaker then will brighten up by itself. Once bright if I turn it off and back on again it is dim. then again will brighten up at some irregular time. I am in a middle of a rebuild an have not got to the stage of figuring it out. Sounds similar to your problem.

Looking at EL-71 in the FSM, the lights do not feed off each other but share a common ground for both high and low beam through the lighting relay. The relay switches the ground between the high beam circuit and low beam circuit.

The positive side runs through the fuse box through connectors to the lighting switch then back through a series of connectors/harnesses to the lights.

If I had to guess I would say it is a corroded or loose terminal in one of the connectors. I would start at the head light connector first then work back.

 

As Genewiz said, they share a common ground. There's a thick connector right under the steering wheel behind the panel. One of my headlights use to be more dim too at random times. I found out that not only was that connector loosely connected, but when pulling it apart you could see that the common ground was pretty burnt up. I would definitely start there.

 

I know this is an old thread but I had a blown headlite fuse and caused passenger light to be dim and driver light to be bright. Also caused the high beam to stay on, on the dash.