99 Dodge Ram 1500 Electrical Problem

What's the symptom or problem?

In years past they applied 12 volts to the high beams or low beams through the dimmer switch, and the common terminal was grounded. On your truck they are applying 12 volts to the common bulb terminals all the time, then they ground the high or low beam terminals through the dimmer and headlight switches.

Caradiodoc

There are two different head lamp systems. There's a two lamp and a four lamp system but I would approach them the same way. Older systems with a common ground had a common problem caused by a bad ground wire. Current would go through the low beam with the bad ground, couldn't get to ground so it would double back through the high beam filament, over to the other high beam, then to ground on the other side of the vehicle. Since the side with the bad ground now has three filaments in series, the bad side is dim and the other side is hard to see that the high beam is on very dim because the low beam is on full brightness and outshines it.

Your system uses a common feed instead of a common ground but I would try using the same approach. That is to unplug the working bulbs, then see if the bad side has new symptoms. In particular, suspect to find the dim bulb is off completely now.

Plug in one bulb at a time and get each one working, then all of them should work properly after that.

Caradiodoc