Thanks for all the help on this. In the end I was chasing the proverbial non-existent connector.

The final result was that I installed a Powerline HD module #31826. Powered it from the rear power port I previously ran to the cargo area. The splices were tricky as I wanted them inside the body just inside the rubber plug. After a 250 mile round trip there are no problems to report.

One observation was that a 5x8 enclosed trailer removes 12+ mpg (average). This trip was a combination of Stop&go (city, up to 45mph), hills (50-55mph) and flats (55-70mph).

-R