On my 30' triaxle flat bed when you turn the headlights on the truck you no longer have trailer brakes. My brake box doesn't even recognize that there is a trailer hooked up anymore. I'm thinking it's a grounding issue somewhere. Everything has worked previously on the trailer. Any suggestions?

 

First I would have a look in both ends of the connector truck and trailer for a burnt/melted pin. It sounds like you have lost the trailer ground so might be a melted pin, the wires at the plug under the truck sure take a beating from rocks so maybe a broken wire there.

 

I put a new plug on the trailer yesterday and it's doing the same thing on another truck, so it's on the trailer somewhere.

 

Check the circuits on the trailer using a 12 V battery and jumpers at the connector. What you're describing about has to be a double fault, or a crazy grounding fault.

Do you really have brakes when lights are off, or does the "smart" controller just think you do? Does anything strange happen when you apply brakes with the lights off, like a turn signal or clearance lights come on dim. You'd have to temporarily disconnect the trailer control brake light wire to test that.

 

If you have regular 1157 light bulbs and not LED's I have seen where the element from the tail light snaps off one end and shorts across the element for the stop/turn element.

 

Run a test wire from a shiny patch of metal on the trailer to a shiny patch of metal on the truck and see what happens.

Is there a junction box on the trailer? Make sure there's no broken wires in there.