My truck was has a trailer hook up but i have not used it. bought the truck used so the previous owner might have hauled trailers. also, the truck belonged to a volunteer fireman and the black box under the dashboard (he had installed) is still hooked up. it has two switches to operate flashing lights (headlights and tail lights on separate switches). i do not use them.
I would suspect the turn signal switch.
Voltage for the brake lights go thru the turn signal switch before getting to the rear lights.
SOURCE: LEFT TRAILER TURN SIGNAL DOES NOT WORK
I'd get a 12vdc test light if you don't have one already & start from plug, and keep probing back from there. It may be as simple as a bad connection under truck where joined into harness.
SOURCE: LEFT TRAILER TURN SIGNAL DOES NOT WORK
I have seen once before that the fuse box was bad on a 2003 Dodge 3500. I had power in and nothing out. We had to replace the box. Not cheap! Hope it is simple like a loose connection though. I think I had to pull the inner fender on the drivers side to access the bottom of the fuse box. Pain in the ****, but...
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