FBC Harness, Trailer lights are not functioning properly

I Purchased a FBC Harness, installed it and have been using it for a couple of months, its Been great being able to back up at night without the glare and the bright lights in the backup camera but I've noticed that whenever I wire up my trailer to my truck the left light on the trailer side always stays on. I think it might be because of the relay in the system messing with something but if someone has any insight on this to help fix to get this working normally that would be great.

 

Left trailer light or left truck taillight? I'm guessing by trailer side that you mean the left trailer light.

Illuminated as in tail (parking) or brake? Brake should be brighter than tail. Some photos of what you observe might be ideal. I'm guessing tail.

Note the relay coil is on the reverse circuit so assuming you’re talking about parking lights or brake lights, there isn’t a good reason to explain how the relay is contributing to the trouble as, during normal operations, it behaves as a closed circuit. One thing worth trying is to disconnect the license plate intercept T portion and observe while the trailer is connected. You could also disconnect the reverse T if you still observe trouble with the license intercept not in place. That should tell you if the DIR harness has direct correlation with the trailer light problem.