Thanks for the response. I don't claim much electricity knowledge but am willing to work at it.
I did get over to the trailer last night and set the TV emergency blinkers on. The trailer rear continued to show the problem, right blinks, left dark. One by one I switched the bulbs from the right side to the left, to make sure that all the bulbs were ok. No change in results.
Attempting to follow richinny's suggestion, I put my 12v
test light on each of the two housing's white ground on the back and then probed the contacts. No test light. I did make sure to test only one bulb at a time as I read that they are wired in series.
My best guess is that the red (left blinker) wire must be bad. Any other thoughts or suggestions?