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Sounds like grounding problem, the Green and Yellow are the turn signal/brake lights, the brown is the running lights, the frame is ground and goes through typically the white wire. If you can't get both the running and turn signal to work at the same time, check all the grounds including on the truck.
The Gen4 trucks which were all incandescent lighted didn't care what type of lights are on the trailer. Each of the lighting circuits came straight off a fused relay in the power distribution module.
It’s usually older vehicles that have that issue. And at that, not all do. I’ve been with U-Haul for 17 years and counting. I’ve trouble shot more vehicles/trailer wiring issues than I care to think about. It’s usually a very simple fix and overwhelming it’s a grounding issue. It’s just a matter of finding it.
What the idiots are doing when designing the software that a truck needs is each year they keep paying so many warranty repairs from customers who hook crap junk trailers to their brand new shiny pickem'-up trucks and they have tried to make the trailer tow circuits so robust to be able to handle every single style of trailer ground, light bulb and dirty resistant wiring they more than likely will have to relax the metrics in the software so that it doesn't see a hazard when the trailer tow circuits goes 1 amp over the rating.
