We just got a new truck and when I tryed to pull my horse trailer the brakes are locked up on it but it pulls are camper just fine. Looked at the plug in on the trailer and you cant take it apart so do I just need to cut it off and replace it? It is the 7 pin not useing an adaptor. Not sure if it matters but the horse trailer is an older one 05 and the camper is 2010.

 

Find someone knowledgable in trailer wiring because it sounds like it (either the truck or trailer) was wired incorrectly. When I got my first flatbed trailer the old boy that built it didn't go by the standard pin configuration but wired my truck the same so it worked fine till I got my second trailer which was wired the way it was supposed to be and it did what yours did. After finding a good trailer repair they had it figured out in five minutes and fixed in ten.

 

make sure the truck is wired the right way 7 pin and a tester, than cut the junk plug off your trailer and re wire with a 7 pin. Once the truck is wired right you can hook up to 99.99% of the trailers out there with out wiring problems. Every new plug comes with a color chart to pin number, very easy stuff to do right the first time. The beneficent of having the seven pin plug is if your buddy's dont have the 7 pin plug they cant barrow and trash your trailer

 

well the truck was wired right took it back to the dealer ship and they checked it out, changed the plug on the trailer wired it like plug said and still didnt work right. had to change the #2 pin with the #4 to fix the breaks but then the lights were all off. Dont understand what happen to it unless someone messed with the box on the trailer for some reason. We hooked it up to my sons 06 and the lights were back wards I just dont understand how it worked fine on all our other trucks but then all the sudden nothing worked right.