I just started having this problem in December 2019. I took my 2016 Ram 2500 to a trailer supply store. They checked it out. they found one of the plugs on the wiring harness broken and was getting water in it. So all of the wiring was replaced. The plug in the bed all the way to the bumper plug. all brand new. it was tested while there and all functions were good. Left the parking lot hit the brakes same thing. It would ding and show the message check trailer brake wiring. Took it right back in. The guy rechecked it and everything he did still worked correctly. He told me it would have to go to the dealer so they can test it. He said it seems that it is in the module in the truck. I'm not sure where that is in the truck. I want to try and figure this out instead of taking to the dealer. I don't want to have to pay a 160.00 charge to hook it up to their computer. And them tell me its the wiring I just had replaced. The trailer brake fuse that's in the engine compartment was reset and still does it. Not sure where to go from here. Hope someone on here can point me in the right direction. Any help will be appreciated. Thank you.