agree, there needs to be a wired grnd connection (white wire) from truck chassis to plug and from plug to trailer. Some trailer plugs don't pass the ground thru and rely on the connection of the ball & hitch.
and sometimes you have to run a ground wire all the way to the studs of the lights (or brakes) and not rely on the trailer chassis as a ground.
A common symptom of bad grnd is that your lights appear to work until you turn on your parking lights and then your brake lights don't work. This is because the circuit is using the 0 volts on the parking lights as a return until you put 12v there, then it doesn't work.
Some vehicles have yellow turn signal lights separate from brake lights. They require an electronic adapter because trailers use the same lamp filament for brake and blinker. When these adapters stop working, weird stuff happens.
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