This may sound like a dumb question, but you do know that there is no 12 volt voltage to the brakes from the brake controller unless some one is holding the lever on full? When you push the brake peddle the voltage going to the trailer brakes can be as low as 1 volt or less depending on the brake controller setting. What brand controller do you have?
Yes poprichie I realize there is no voltage without the brakes engaged. My thought was, is there a loose ground connection tied into the magnets. My brake controller is a Prodigy P2. It is almost like an intermittent issue, sometimes it works fine.