According to the factory wiring diagram 8W-51-4,
the third brake light is on the same power circuit (+) as the corner brake lights, but they do have different grounds (-). Third brake light is on grd. G303, both corner brake light bulbs are on the same ground circuit, G304 (8W-15-18)
According to the WD (8W-15-18), the rear license plate lamps, and the rear power outlet are use the same ground circuit as the corner brake lights..
So check these other items and see if they are working OK, if not you have a ground problem..
WD as follows:
Red is power to bulbs, green is ground to bulbs...
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