It would appear that the tow vehicle is sending a "hot" 12 volt
current to the trailer that is in turn locking the brakes on. The hot wire (12 volt) and the brake wire could have been reversed by a previous owner. ( on the trailer)
But to be sure, Start by checking the leads from the female plug end on your tow vehicle to make sure they are wired correctly. Eliminate one problem source at a time.