It sounds like you have a ground problem most likley on the trailer. I would check every ground point and make sure the conenctions arent corroded and try it again. On a few spots on the trailer you will find a white wire with a ring terminal screwed into the trailer. One will be under the tounge, the others possibly at the rear near the lights. Those are the grounds. I would remove the screws, clean the terminals and connection point on the trailer and reinstall with a new screw.