I had the same problem mate, I checked the sunroof drainage holes and made sure they wern't blocked, then i took the doorcard off and checked the membrane behind. Sometimes the membranes shrink with age and this creates small pinhole gaps which allows water through. I replaced the membrane with a new one from VW and i bought the sealant that comes with it. Since replacing i havent had anymore leaks, so its deffo worth checking your membrane out.