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99% OF TRAILERS have no fuse or breakers between truck and running/tail/brake lights and rely on the the truck to protect those systems. The most common failure is where the trailer cord goes into its first distribution box right at the fifth wheel pinbox, second is that the cord end has got hot and doesn't connect correctly any more to the truck plug. From there it could be virtually anywhere in the mzarker/running light wiring. Good Luck