Turn signal not working with lights on

[akf15e]

OK, next issue, now that I have learned not to put the battery in backwards! I have had an issue for the past few months with the left turn signal on the rear of the Monty. It works fine with the haedlight switch off in my truck. When I turn the lights on, the signal no longer works. My troubleshootign has included cleaning the 7 pin connector, switching bulbs from left to right signal (left still fails), and new bulbs. Any ideas? Thanks in advance!