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Trailer wiring color codes are different from marine/boat wiring. White is always the ground (neg), and for fixtures is simply attached to the metal of the trailer frame, usually within the fixture. Green and yellow are the brake/turning circuits for each side, and brown are the running lights (tail lights).

The white ground comes out of the vehicle, through the plug, and the short white wire is connected to the trailer frame. Then the trailer frame grounds all light fixtures.