Hi,

Mine is A6, 98. I purchased the car 2 months back & everything was working properly until one day the 'Brake light fail' ( Crossed Bulb symbol) indication started flashing in the dash board. I took to near by service center & they changed few bulbs & things were quiet good for few days & started again.
Again I took to them & they said they can't diagnosis as this is an intermittent issue.

I took to a Audi service center they checked the bulbs & to told the bulbs weren't in correct voltage. They changed few bulbs & worked properly for couple of weeks & started again. Took to them again & they also told me as this is an intermittent issue & asked me to wait when it occurs permanently. Audi service people doubt this is because of Brake switch or socket, but they can't conclude properly.

Note: The indication comes only I press the brake pedal.