doortop seals, "U" shaped channel holds mold..
I was starting to clean my car for Spring. And was juicing (with silicone) all the rubber parts on the doors. When I (for the first time) realized the top of the door seals are not solid, but "U: shaped and just overlap. So the channel inbetween the two parts is a perfect place for mold to grow.
I cleaned them out with some silicone on paper towels. And am certain some black mold was growing in there,
So if you hate black mold as much as I do, You might check your doortop seals
The two parts overlap, so it looks like it is sealed. but if you start at the end, You can see the flaps are just an open pair of vertical flaps laying one against the other hiding a "U" channel.
All four door tops on the Mk3