The seal that hold the window regulator to the door behind the door panel. Either the front or rear one as the water will always run to the back. Basically it involves removing the door panel and re-sealing either with a new seal from vw or some butyl strip off ebay. Don't use silicone thats for sure.
Have a wee search as its common and covered plenty times here.
PS - You can usually tell if its the seal if when you open the door and there is water lying on the door step. Should be leaking out between the door panel and door.