The brake light warning indicator - went on and off for a few weeks indicating a problem with the driver side brake light on my 2000 A6.
Then it went on for good so I replaced the brake light bulb. This did not solve the problem - the brake light still does work when depressing the brake pedal. I double checked the bulb and all connections - they seem to be all fine.

Passenger side brake light and all other lights work fine.

Could this be:
a) tail light bulb holder
b) brake light switch
c) something else?