Hi - can you help?

I have a 2007 2.0tdi 5dr A3. It has auto everything, full electrics and xenons. Recently the high level brake light has started to be very dim. Sometimes it works fine, other times it is so dim you hardly notice it is on. It comes on when the brake is depressed so appears to work fine.

All other lights are functioning well.

If you start and run the car when it's dark, it seems to work fine.

When the car runs during the day, it seems to be dim.

Has anyone got any ideas please? I have already had to replace the turbo, do the ABS service kit to get rid of the ESP light all 3 months ago so this new problem is really annoying - I don't think this is related!

Thanks.