[Carl n Susan]

I occasionally get the same message, usually when it is wet out, with or without the trailer attached. It even happens without applying the brakes. I suspect it is a loose wire in the plug or a short due to water intrusion. There was a recent post on one of the Ford forums about the "Trailer Fault" message. Supposedly there is a Ford TSB relating to the plug allowing water to enter. The poster also said Ford's diagnostics can pin-point the specific cause of the message. Of course if it is the trailer, it won't be covered under warranty (assuming you still have one).

I am planning to have it checked at the next service interval. Since it doesn't fail all the time, I doubt the techs will be able easily find the problem, but hopefully the diagnostics will give them a hint.