First, I attached the entire exterior lighting schematic below for you to see. I had to cut each of the two pics in half to make them readable, but I did overlap them so you can follow from one to the next.
Now, it can't be a fuse because the third brake light gets power from the same source. It can't be ground if other tail lamps are working because they all share the same ground. So, that leaves either burned-out bulbs or an open circuit in the wiring.
If you look at pic 4 specifically, I highlighted the wires, lights, and a splice. I also circled the splice. If you can, first confirm it isn't the bulbs. If they are good, check for power to the Green/Black wire going to the lights. Here is a link to help:
If there is no power to the sockets, then chances are there is a break at the splice I circled. You will need to follow the green/black wire back until you come to the splice to see if that is broken or corroded.
Try this and let me know what you find. Also, let me know if you have other questions.
See pics below.
Images (Click to enlarge)
Tuesday, July 20th, 2021 AT 9:33 PM