Left rear turn signal problem

All lights and turn signals work fine with headlights off. With headlights on, the left rear turn signal blinks very weak, however, the left brake light also blinks, brightly. I’ve inspected the wiring harness front to back, no visable chafing. Am considering replacing the selector switch. Anyone have any suggestions. Other than that, it runs as good as the day I bought it new in 2003 for $13,500. Thanks for your help. Jim

 

No, just original bulbs. I tried replacing the bulbs, no luck

 

The taillight harnesses are the same from side to side. Remove both taillights and unplug the harnesses down near the frame and switch harnesses from side to side and see if the issue follows. If it does then and new taillight harness will fix your issue. If not then if you have aftermarket trailer light wiring that'd be a good second place to start.

 

Thanks, good info. I’ll switch harnesses. Jim

 

Make sure the bulbs are all the way snapped in, I had an issue with my blinkers. I didn’t push the bulb in all the way