Do you have a 12V test light? If not you should. This wont be the last time you will need it. Check youtube.com for lots of videos on how to use one. First rule of troubleshooting:1. Divide and conquer. Disconnect the trailer at the truck, then with a buddy operating the brakes turn signals and lights in the truck, you will connect the alligator clip of your 12V test light to ground and probe each pin in the connector on the truck. I would as BCB mentioned look specifically at the Green wire but it may be to your advantage to test each pin to understand what each one does. ( write it down) In an LED tail light, when the brakes are applied it usually just turns on more LEDs to increase the brightness. If you have power from the truck that is the same for both sides,( one connector powers both sides) Typically ground, right and left running, and right and left turn/brake 5 wires total, maybe 6 if you have a backup light. But this will tell you its not the truck. Reconnect. Keep the buddy in the cab. Second rule of troubleshooting. 2. Substitution ( compare with known good) At the tail lights if you can pull each assembly out or reach behind you can probe through the wires on each side again with the buddy in the truck again going through each operation. Youre mainly focused on that brake light condition so again as BCB has identified its likely a green wire at the light. If when the brakes are applied probing this wire, you get no light on your test light, you have confirmed that you do not have power to that point, you will need to find the point in the wire by probing where its either open or grounding. Its almost always at connection, or areas where it can get pinched, or where the wire insulator can be stripped away from wear, so look for it. If you do have your 12V test light come on at the light, you will need to replace that tail light assembly. Good Luck, Steve
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