I dont know If I am talking about the same thing as you but I recently changed my blower motor assy. in my 300e. The flap that puts the system into recirculation mode had alot deteriorated foam on it that would turn into dust if you touched it. Well I pulled out my trusty shopvac and got all the foam off of it. There was still some foam in the system and when I turned it on for the first time my dash was covered in little tiny black particles that probably came from that flap.

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Adam,
2003 Volkswagen Golf GL 24,000 miles. 1.9L I-4 TDI 5spd.

1986 300E. 115,000+ miles. 3.0L I-6 1st daily ride, around town only. (Gone)