travel, not to many replies to your question as yet. If you feel the area in question does it have a powdery surface, can the paper be moved over plywood surface and does the paper lift from plywood surface? Is this area at the lower bottom left of the drivers window?
If its being caused by a water leak the outside of your window frame should be checked for sealent along the top of your window frame first than the sides, the window can collect water inside window frame because of poor seal at the center of your window water will collect in track of window inside and the slots outside of window could be plugged with dirt or dead pieces of leaves so water can't drain from window tracks outside.
Let us know what you find and we will go from there.

"007"