Thanks Kevin. It seems that someone must have covered this access panel up or for whatever reason there isn't one on this particular trailer...but at least now I know the general location of the harness...so that is where i'll look.
I did exactly what you suggested with the original back plate cleaning etc. I even got ahold of another back plate in better condition and tried it. Still no luck in getting this thing to work. But, since the red wire is the likely culprit I am hopeful that finding the harness will reveal a short. If that doesn't work then I'll have to rewire that fixture since I know the fixture itself is really not the problem. The whole issue of ground is strange since there is clearly power going to the light socket and is certainly got a good connection to the trailer skin...so what else it could be I'm really unsure of...
Thanks again for the suggestions! I will keep the forum posted on how this resolves for others who find themselves with this problem