Truck has turn signal everywhere (truck and trailer) when Hazards are on. Right side markers all work fine when switch is flipped to the right. When you switch it to the left all the lights flash but the rear of the truck. 12 volts to the bulbs on rear left when hazards are on and 0 volts when switch is flipped. Just figured out that the left brake lights do not come on as well on rear left.
I was able to figure it out from an older post. It was a fuse that was not in the correct slot in the fuse box under the hood. G4 fuse. Turn Signal relay location on a 2013 Petrbilt 386 Thanks @Goodysnap
I would first disconnect the trailer plug and see what happens with the left t.s. Check the left rear t.s. bulb. There is a chassis node with 3 plugs (21 pins each) that is either on the frame rail or under the driver's side battery box. The rear turn signals and brake lights pass through the node. Follow the harness at the rear toward the front to the node. There will be at least one connection (2 harnesses from node to rear lights). Separate that and see if you have a left t.s. Probably a stud on the frame rail for the ground near the rear of the truck, check that. The problem may be the t.s. switch.