User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.2) Gecko/20040804 Netscape/7.2 (ax)
FWIW, the ENDS of the cables appear in the receptacle boxes, so at least
you have test points to isolate where the discontinuity might be.
Unplug the trailer and work with a multi meter or (better IMHO)
flashlight continuity tester.
Matt