No. I mean that there might be a pinched or exposed wire somewhere along the harness. that brake light wire could be pinched or making contact with a wire next to it within the bundle that might have a constant 12v feed. It could also be making contact behind the fuse bank or relay bus bank. Something is feeding it 12V constant. I would be a bit worried because if you have touching wires somewhere, it could be a fire hazard.
Has the owner installed a sound system or any other after market parts? Is the car leased or Financed? if so, take it to the dealer and tell them about the idea of a pinched or touching cable as it is the only logical reason for this problem. If the dummies there don't know what you mean. Find a shop that specializes in car electronics and wiring, they should be able to cut that wire to the brake light and run a new wire bypassing the contact point.
VAGCOM allows you to plug up to your car and allow you to manually activate head lights, breaks, blinkers all from the laptop. And it might even give you a fault code for that brake light, such as voltage intermitted plus error code.
EVERY AUDI OWNER SHOULD BY A VAG-COM UNIT!!!!!!! A MUST!
the best investment he can buy.
It would be hard to find where that wire maybe making contact with the constant 12v.
If it's just 1 break light, it can't be the brake switch on the peddle, nor sensor. Make sure it isn't the rear fog light you are talking about.
If you buddy is savvy with reading wiring diagrams and schematics, buy the bentley book for his model car. back in the day when I used to install car audio, there was a site that gave you all the wiring color codes for all cars for troubleshooting.
Best of luck. I hope you understand what I just wrote, in a bit of hurry so typed it fast.
before, people were really helpful on this site, but now....... doesn't seem that way anymore.