Intermittent failures in electronics can drive one insane to trouble shoot. There is a reason why connectors used in military equipment and aerospace applications cost several hundred to thousands dollars each. Yes, there is a reason for all those Mil-Specs.
Luckily, this trailer wiring is not too complicated if one really wants to track it down. But perhaps the simplest thing is to rip it all out and to start over.
"Old age is the most unexpected of all things that can happen to a man" -- Leo Tolstoy