Hey Blue
Who knows how much VW would rip you off for a bit of polythene!!!
If you want to fit some new membranes get yourself some really decent double sided tape (maybe carpet tape) and a sheet of polythene from a builders merchant.
Fit some double sided tape around the outside edge of the door and around all the edges of the holes. Roughly cut a piece of polythene to size and stick it on the door. Then make a slit for all the controls (mirror adjustment etc), push it firmly onto the tape all round and trim the outer edge to size. There you have it perfectly adequate door membranes!
Water gets into the door in the first place due to worn seals along the bottom edge of the moving windows. Even with a really good seal it's inevitable that some water will get in when the window is wound down. If you look at the bottom edges of your doors there are drainage holes to allow any water to escape.
Hope this helps
Toby
