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In most case if the third brake is working then the brake switch is working properly and in most case the problem is in the multi-function (turn signel) switch.
Check the brake light bulbs and replace if needed.Check the voltage on the Orange/Black (O/BK) wire into the multi-function switch. It should show 12 volts when stepping on the brake pedal. If it does not, repair the circuit between the Brake On Off (BOO) switch and the multi-function switch. If there is voltage on the O/BK, use a jumper wire to jump the O/BK to either the Blue/Black (BL/BK) or the Blue/White (BL/WH) wires. If the brake lights come on, replace the multi-function switch. If not, repair the circuits going to the brake lights.