I had the same light and turned out to be the ground on that line. Check to see if your middle or third brake light is working. If not, check the ground for that wiring series. My ground terminates in the trunk. Once I regrounded the circuit, all was fine.

 

my middle light doesn't work i think but when i hit the spoiler it flickers. also where do the wires run cuz it looks like a pain to try and remove the spoiler. when i tried takin off the plastic panel on the hatch the clips would stay in the trunk but the panel would slide off the end of them