I did this project yesterday. Mostly because no matter where I directed the air via the buttons it came out all vents almost equally. The front flap had no foam left at all. The rear flap still had most of the holes covered with a super thin layer of brittle foam. So I removed what I could from the rear flap and covered them both with silver foil duct tape.
I plugged the HVAC controller back in to make sure the flaps both worked before I put it all back together, i'm glad I did. The rear flap wasn't working. It must have fallen out of the actuator while I was removing foam or taping it. Well I got it back where it belongs and everything works great.
Now I can get the air to come out of what vents I want and I think the AC blows colder now that it isn't picking up any heat from the core.
1998 Porsche 986
Threaded Mode
