Hi,
If the third brake light is working and the bulbs are good, you need to check a couple things. If you look at the schematic I attached, you will see the brake light wiring. I have arrows showing where the power comes from and where it is going. Note that the third brake light gets power from the same wire as the tail lights, so if they aren't working, either you have a bad splice in the wiring (which I circled their locations), or a bad ground for the side lights.
Here is what I need you to do. Note the power supply for all brake lights is a white wire with a tan tracer. I need you to locate that wire on each side and check if there is power to it. If this happened all at once, I suspect the first splice is the issue or the ground. But we need to check for power at the lights and work backwards to see if we can determine where power is lost.
Additionally, if you find when using a test light or volt meter there is power, you will have supplied a new ground. In this case, check the black wire for continuity to ground.
Sorry about the sloppy writing on the schematic GRD is the ground. I can't write with a mouse pad. LOL
Here are a few links you may find helpful:
https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester
https://www.2carpros.com/articles/how-to-use-a-voltmeter
https://www.2carpros.com/articles/how-to-check-wiring
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Let me know if you understand what I am suggesting. If you have questions, feel free to ask.
Take care,
Joe
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Wednesday, April 8th, 2020 AT 5:33 PM