2016 Peterbil 579 Marker Lights won't work.

Shouldn't have anything to do with the trailer. Either they're on a spare switch that isn't labled or you have a wiring problem.

 

Older trucks it would be on its own switch. These new ones I don’t know.
@pushbroom @Goodysnap

 

Similar set up on a 95 379.

Drill and screw into the stainless in an inconspicuous point, and connect a ground to the frame.

I discovered my ground fault when the diesel nozzle would create a ground from bar to tank.

If it’s not ground, then trace wiring, check all fuses.

 

Just test my idea with a metal rod or clip connectors on a wire - see if that might be the problem.

 

Factory wired switches in the TMC trucks are labeled “AUX” right side of the dash.

Also key must be on.

 

You’ll either have spare switches by the ignition key or over to the right by the air knobs. Turn the key to the on position and start flipping switches until they come on.

 

With the key on?

If so then look for a fuse in the panel by your foot that isn’t labeled. If the fuse is good then head outside the truck like mentioned above.