I had the same problem. The source was the Windshield Seal. You could replace the windshield seal although I have heard it can still leak. I opted to put silicone (clear) sealant around the sides of the seal and the bottom after cleaning thoroughly of course.

If you take the tip of the silicone tub and put it between the windshield and the seal it seems to work fine.

This was my experience. hope it helps you out.

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83 TD wagon "Madge" PO's given name. 410k (150k engine)