I had the same issue, and for months thought that it was my truck. I had a Nissan Titan at the time, and had to purchase a couple of relays from the dealer since the truck didn’t come with the Big Tow package when I originally purchased it. I was so angry at the truck for the issue, and had tried everything (testing solenoid, changing fuses, etc), that I went out and bought a Duramax, against the little lady’s wishes. Got the new truck home, plugged in the 7 wire round, and tried to back up the boat…guess what, brakes were still locking up. The only place I didn’t check was in the round plug itself. The wire that handles the reverse lights was hanging on by one strand of copper. I rewired the plug, and voila, the boat can now travel in reverse. Needless to say, I did not and have not to this day told my dear wife of my mistake. She’d want me to get rid of the Duramax if that was the case, but I’m having too much fun with the power to let her in on the secret.
.
