If I had to guess, I'd say that there's most likely a short circuit in the converter box. You can confirm this by disconnecting the 12-volt connection at the positive battery terminal that leads to the converter box. If this fixes the problem, it confirms that the converter box has shorted out and needs to be replaced. You should be able disconnect the connectors by pushing on the release tabs then pulling the connection apart.
For a replacement, I'd recommend the Tekonsha # 118545 T-One... view full answer...