I'm at work today doing some maintenance on our trucks, and a tad stumped. Maybe my head is just mush, anticipating the holidays and not really wanting to work today. :-)

So, we have a Ram 3500, that had a flatbed installed a few years ago. Apparently, it wasn't wired correctly from the start.
I have NO diagrams of the flatbed assembly, so I'm going in to this blind.

The flatbed has the combo brake/tail/signal lights in the back, multiple marker lights around the sides, and the up rail assembly has two LED (3wire) lights up top that also serve as brake/tail/signal lights.

The problem is: With the lights on, all lights work, except the driver's side tail light. When you hit the brakes, the taillight works, but the upper LED lights go out. Using the turn signals, there is no issue.....all lights flash properly.
It should also be noted that this truck has a trailer brake controller....which works fine.

Any thoughts?

 

Bad ground on the one that goes out and/or the left turn light. Odds are it's the same ground.

 

OK.......scratch what I said about the main light in the back being out.

It's just the upper LEDs that go out when you hit the brakes. They work when using the turn signals, and just general running lights.