Your newly-added trailer brakes use a totally separate circuit than the lights on your trailer. But I agree that a bad trailer ground would have been my first suspicion too. Make sure your main trailer ground is made to a clean bare metal spot on the frame, one free of paint, primer, rust, grease etc.
I suggest you test the vehicle and trailer separately since this will isolate the issue to either the Ram wiring or the trailer.
Disconnect the trailer and apply power to each of its circuits... view full answer...
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