Anyone use X5 to put a boat in water?

[taosx5]

I have an 03 X5 with an e-trailer hitch and wiring harness. I installed both several years ago and have never had an issue. We have a new boat trailer that automatically applies the trailer brakes when the trailer is not plugged in to the tow vehicle. If it is plugged in then the brakes are not engaged when it goes in to reverse.

Today was the first time using it. We hooked up to the trailer (boat off and in water) and it worked great ie brakes not applied on trailer when in reverse but unplugged and in reverse trailer brakes are applied. We took the boat out of the water to go gas it up and then put the boat back in the water. The launch worked great, no issues. After we put the boat in the brakes on the trailer kept engaging when I would put the car in reverse indicating the trailer harness does not recognize it is plugged in.

The wiring harness did get wet when we put the boat back in the water. Thinking it might have been a blown fuse I checked all fuses that I thought it might be but did not find any. The lights seem to be working correctly on the trailer. I checked the tow vehicle wiring harness with a test light and did not have a hot wire but when the lights or brakes are applied I had power.

Anyone have any ideas on what might cause the brakes to engage on the trailer while the trailer is plugged in and tow vehicle in reverse?