Hey, I had the same problem with my drivers side door (lupo Gti), I ended up replacing the door lock mechanism...dont have the part number to hand but think its around £80.00 so not cheap., my mate at VW says they tend to keep these parts in stock as its quite a common problem. Theres a small micro switch in this part that lets the car know when the door has been opened and its this that goes wrong. Unfortunatly no cheap fix, I decided to fit the part myself as my local VW specialist quoted around £150 to do the job, plus they could'nt guarentee it would fix the problem. It may be worth spraying WD40 into the lock to try and clean out any water, (Tip: Dont clean near locks and handles with a pressure washer! Think this is what ****ed mine) I took pictures of the job so i could post them along with the part no if you need. I suggest it might be worth getting your local garage to take a look as there input should be free, plus they might be cheaper then the places near me? Check it is only your passanger door with the problem, if its the micro switch when you open the problem door it also does not activate the interior light? Hope this is the start of the help you needed?