Stupid question

Tractor front and side trun signal and trailer turn signal are on 1 circuit

Trailer brake lights are on the brake light circuit.

The tractor tail lights are the only place they merge

 

How many wires does a 4 inch round light plug have? 3. 1 is ground. 1 is tail. And 1 is brake and turn.

 

Possibly a bad grounding wire, can be checked simply using a test light probe to ground the light to the body.

 

Doesn’t your truck have a test lights function? Not sure if most do or not.. mine does

 

Does it do this when disconnected from the trailer? Does it do it on the right side also, or just the left?

If it only does it when connected to the trailer, check for a bad ground or a faulty connection (corrosion) that's creating a lot of resistance.

If it does it when disconnected, try changing the bulbs out.

 

That's a relatively new thing. And imo a completely unnecessary and complicated thing that will cost more money and problems in these looking run than it ever saves.

 

I've had it on my trash national... Now 2020, and previously I believe 16 & 17.....
So maybe not so new?
No problems and I missed it on the Freightliner...

 

16 was only 3 years ago. That's pretty new.
Check back in 10 years when corrosion has had a chance to set in and tell me how much you miss it while your writing the checks to fix that unnecessary system.