Clearance/tail marker lights on the trailer keep turning off.

I don't doubt for a minute that some "technician" told him that. Too many young "6 whiskered kids" in these shops that plug in a device that says XYZ, and tell the customer XYZ is the problem with your truck/car. Nobody has the ability to troubleshoot a simple problem anymore. As mentioned above, START SIMPLE! I agree with the ground wire problem. Makes the most sense. Too much "voltage" from the truck for an "old trailer". Lol!

 

Folks who understand electricity will know that a source (ie the truck) cannot "force" extra power into a circuit. A circuit only takes as much amperage as required since amperage is a factor of voltage divided by resistance. High(er) voltage will not cause lights to go out and come back on.

If the voltage was high, well the truck would be upset and throwing all kinds of faults. Probably blow some lightbulbs too. I mean its a 2020 Shaker that OP is driving. Everything is computerized. Guarenteed there'd be a warning message or fault displayed if the alternator was producing too high voltage.

 

I had this problem too. I thought my electric cord was bad, so I bought a new one. Didn't fix it. I went to a repair shop and they found the problem...

Where the tractor electric line plugs into the trailer, there is a plug on the back side of the trailer connection that becomes corroded and can come unplugged on its own. They plugged it back in and I was good to go. There's just 2 screws that hold it in

 

You are absolutely spot on with your response. Thank you!

 

Is this is always happening with one trailer. Or multiple trailers? Some trailers have a circuit breaker on the trailer side of the light cord. The idea was if there is a dead short in the trailer the circuit breaker will trip in the trailer instead of in the truck. They will reset when they cool off thereby making the trailers lights go off and on. Something to keep in mind.