Searching on another forum has brought me to the conclusion that it is my Brake Light Failure Switch, part #24 in this diagram...



Symptoms
-Only my third brake light works
-'Brake light' indicator NOT LIGHTING UP on my dash

Now my two questions for you guys are, 1) Is this definitely the problem? and 2) How the heck do I get to this part?!

Any and all help would be deeply appreciated asap, I would hate to get pulled over for something so trivial

 

Can you test for a good ground on the tail lights - the two outer share the same (G601)

 

All bulbs work, here is a video of the same exact issue I am having (click it)

 

Your bulbs are dual-filament. Have you replaced the bulbs yet just to rule that out?

 

^This was one thing I almost overlooked, as I am coming into this car from a 5th gen. I inspected both bulbs and both filaments were intact. Switching them between left and right also showed no change.

 

Super late on this diagnosis after fixing this issue, but I figured it could help someone else in the future.

My Brake Light Failure Sensor failed. This part is located just under the left tail light bulb sockets behind the trunk plastics (#24 in the diagram), and replacing it is about as easy as tying a shoe.