pezzy is exactly correct. The flap or baffle that regulates air direction is lined on both sides with the foam. I had the same thing happen with my '92 400E.
Caution: PICK IT US ASAP WHEN IT HITS YOUR INTERIOR. The stuff is very sticky and deteriorates upon contact. It will stain everything it its. Lexol cleaner gets it off the leather fairly well. Carpets...that's another story. Just pick up what you can. Avoid scraping the carpets with vac attachment. You will just grind it in. Spot clean first with a good carpet cleaner, let dry, then vac.
I had to fix a "ticking" in my fan blower so I removed as much of the foam from the source when I opened the blower assembly. Not a bad job. Just very very tedious. DO NOT GET INTERUPTED. There are a whole bunch of plastic parts, screws, etc that must be tracked and re-aligned upon re-assembly.
When I exposed the "flap" inside the assembly, I removed as much foam as possible by hand. I then used a knife to pry the tape backing from the aluminum flap which held the foam. With care and lots of patience, I was able to get rid of the loose stuff.
Good luck....and let me tell you...Chicks hate that stuff blowing all over them :-)...sounds like you already found out.