I had a similar problem a month ago when I had a 7-pin trailer connector installed in my new truck; when I hooked up the trailer and plugged it in, the trailer wheels were locked up. I took the truck back to the installer; it turned out that they had originally mis-wired the connector. They corrected the problem and I was able to move the trailer.
I understand that mistakes can happen. However, if they had taken the time to test the circuits it would have saved both the installer and me a lot of time and frustration.
Mike
