Ok...A bit late to this thread but clearly a very hot topic here and in Europe.
Apparently VW recognise this as a problem but have not issued a universal recall...in my opinion a very shabby way to treat customers who have mostly shelled out north of 70k on one of their vehicles. We had the rear high level light replaced with a non OEM tail light and video camera a couple of years ago. There has not been an obvious leak issue until this last month. I thought maybe a problem with the replacement part but on inspection of the removed OEM light and the replacement it is obvious that both rely on foam tape to effect a water seal. The same sort of foam tape you use to seal a draughty window. Wholly inadequate to make a water tight seal on a horizontal surface. In other words a completely useless design solution from VW.
Clearly ALL rear high brake lights "sealed" in this way will eventually leak.
So it appears that VW require their customers to notice catastrophic water leaks (delamination of cupboard material, water pouring out of bedside lights, water appearing through the A pillars or dash cup holders ets) before doing anything about it.
The anything (from German California owners groups) is the use of highly suitable Iso butyl tape to reseal the rear light. Since there is no official recall, and my vehicle is out of warranty it looks like this dry weekends job is in my diary!
I really think VW need to fess up regarding the early Grand Calis, get them all back in and sort out the numerous issues that they hadnt thought through carefully enough....sink tap, waterproofing of shower room, software on the control unit (still playing up on mine)...and so on.