On my 2003 dodge ram 2500 truck the left side signal and brake light on my trailer and trailer conector will not work. When hooked up to my truck. The lights on my truck works fine also checked the fuse under the hood. What could be the cause?

 

I will move this to the 3rd gen section for you (2002-2008 1500s, 2003-2009 2500/3500s).

Has it always been like this, or is this a recent development?

 

I would suspect it's the wiring with the trailer that is nackered. Check the plug on the trailer to make sure the wires or connectors aren't corroded. It could be that, or either the wiring throughout the trailer could have a break in it, which would be a PITA!!

 

Sounds like the Totally Integrated Power Module (aka the under hood fuse box) Left trailer turn/brake relay is bad. And you cant just replace the relay either as its "totally integrated" into the the fuse box. Why couldn't Dodge just put a replaceable relay in the fuse box for the trailer turn lights.

 

Just want to add that you should test the 7 way and 4 way connectors to see if the left turn/brake is working on the truck end.

on the 7 way connector Pin 5 (3 o'clock) is the left turn/brake on the truck end
on the 4 way connector Pin 3 (yellow wire) is the left turn/brake on the truck end

I have seen the the 10 pin connector on back (behind the bumper) of the 7 way plug not full seated and was causing intermittent problems with the trailer lights and brakes. But the most common cause is a bad relay in the TIPM

 

I agree with Houseape's first post. I had the same exact problem. It turned out to be the flex circuit panel under the fuse/relay block. I just hard wired into the truck wiring no more than I tow something and I haven't had a problem in 2 years. I also put an 20 amp in-line fuse from the truck wiring leading to the trailer wiring.