Hi:
I just went through the schematic. The left brake light and the high mount brake light get power from the same place. The right brake light gets it from a different place. With that in mind, the idea of a fuse doesn't make sense. However, I did take notice that both the high mount brake light and the right brake light have a common ground. The ground for the left one is in a different location. Therefor, I suspect the left ground is good (allowing one light to work) and the right ground is bad (causing the two not to work). At least that is where I would start.
I attached a pic for you from the schematic. Check the ground I circled in the pic. The ground shows in two different places on the pic, but note the ground number (g402) is the same for the two lights not working.
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
After you check the ground, see if there is power to the light socket. If there is when you use a test light, then you know it's a ground because the test light will be grounded in a different place.
Let me know what you find and if this helps.
Joe
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Tuesday, April 13th, 2021 AT 8:12 PM