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You said you checked the ac drains but you will need to pull the little nipples off of the ac drain ends under the car so they will be freed up. Also open the sunroof all the way and located the drains in the front corners of the sunroof pan. You will see little holes in the corners. Fill up a small cup of water and pour it into the front corners so it will drains into the holes. Look behind the front tires because that is where the water drains out at. If you see if flowing good then it is ok if not then you will need to try to blow the drains out with compressed air. Also when water tested if have found that sometimes just spraying it isnt enough. You may have to let the hose run over the car for anywhere up to 30 minutes to find the leak. So just somehow prop it up over the top of the car and let it run. Most you may also have a drain hose leaking where it fits on the drain tube on the sunroof pan which will require pulling down the front of the headliner to access and securing a zip tie on the rubber hose to tighten from leaking.
2003 Jetta left rear window blue plastic part of the