If there is no water intrusion than you either have a wiring problem or module problem.
Do you have a volt/ohm meter?
Here is the pinpoint test for the trailer fault. It is easier if you have the trailer brake module scanned for codes.
NOTE: Make sure the trailer brakes are electric-magnet actuated drum type brakes before proceeding with this pinpoint test.
NOTE: Every 4 seconds a voltage pulse is applied to circuit 43 (DB) to determine whether or not a trailer is connected to the 7-pin trailer tow connectors.
NOTE: Battery voltage must be greater than 10 volts.