Audi center brake light

I'm guessing you mean the collision avoidance/third brake light. The exact procedure will depend on which A6 you have. On the station wagon, the light is located at the top of the rear hatch. Open the hatch and remove the plastic trim panel to access the rear of the light housing. On cars equipped with a trunk, the light is on the trailing edge of the trunk lid. Open the trunk and remove the trim panel to access the rear of the light.

Actually I found the light was accessible from inside the car. The back cover pops off and the light bulbs come out individually.

Ahh, mounted on the rear deck. Thanks for letting us know you got it figured out.