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Hi, my name is***** from the brake light switch has to go through the stop lamp suppression relay when you apply the brakes. If the relay is bad the brake lights will not work. You should have 12V at the black wire going to the brake light switch. If you connect the black wire to the black/yellow wire your brake lights should turn on. If nothing happens when you jump the two wires together I would replace the relay. I'm assuming that you have checked the light bulbs.
Let me know if you have any questions.