[Gelbster]

There is an easier version of accessing the flap with the deteriorating foam.
If you Search ,you'll find it. The technique requires removal of the Steering Wheel and then some delicate surgery to some plastic part that obstructs access to the flap.
The flap gets recovered in 3M Flue Tape(Home Depot)
This will get you started:
http://986forum.com/forums/general-discussions/67109-small-foam-pieces-blowing-vents.html
http://986forum.com/forums/performance-technical-chat/42802-interior-vent-debris-post323947.html
https://rennlist.com/forums/996-forum/822434-air-vent-debris.html
http://986forum.com/forums/performance-technical-chat/42802-interior-vent-debris.html