I have a Audi A4 B5, several of them.
I did this blend door job on one.
It's caused by degraded polyurethane foam sheet on the blend doors, it's the same sort of foam that holds the roof lining up, which is why it's falling down.
And around the cone on the wagons subwoofer cone, so that's not gonna work properly either. It's a twin coil speaker & the best replacement I can find is a office PA speaker with 2 voice coils, it's a bit bigger, but will fit.
Ok, nah, to do a proper job there's a blend door at the back of the unit that mixes the hot & cold air, the vent that does up/down is plastic with only a bit of that foam to seal it.
IF it's the same unit as the A4 B5 1997-2000.
I took mine out, there's little crazy to replace clips that hold it onto the aircon box, & you have to cut the carpet under the box & peel it back, and remove the flap control motors & a bugger of a screw on a metal bracket on top of it, to even start getting it out.
The first one was a real pain to get out, but I had left the motors on, and not nearly as bad to get back in afterwards.
But the 2nd one was easy to get out, and I haven't figured out how to get it back in yet.
There's a set series of movements, positions, rotations & shifting, but I just can't figure it out.
Somewhere I read that normally they take the whole dashboard out, but that nuclides steering wheel stuff & fukkit I'm not doing that.
Even if I have to start on another car just so I can figure out how to remove it, so the movements are recent & remembers or.
I have that one to put back in & another 2 cars to do soon. Plus another few more in time.
I sanded back the sheet metal flaps & used Super F2 contact spray & some closed cell foam sheet I got from a craft shop.
A layer of glue on the metal & one on the foam, let them dry for a day, another on both surfaces & press the sheet onto the panels & weigh them down in place, or use wood prices or books & G clamp it together, for a day +.
You need some overlap because the flaps not 100% filling the door hole it's in & it needs some suppleness to seal against the edges, so overhangs necessary.
Use silicon grease on all hinges & flap arm controls.
Since it's a pain to do, invest a few $$ & get a new heater core. In the common inlet/outlet end, there's a wriggly plastic divided between hot & cold pipes.
Since it gets hot it gets brittle & I found chips of it that had broken off, so the hot/cold coolants mixing a bit there.
That car, a wagon, i had bought with "A blown headgasket'" cheap.
There was oil in the coolant & coolant in the oil, it was all like funny clay or chocolate milkshake.
For some reason, as soon as I saw under the bonnet, I though 'Oil Copper' & checked it, to finding had blown, corroded thru, and mixed coolant into the oil.
Lots of flushes later, and a 2nd hand oil cooler & it was running great.
For 4000 kms..
And the oil pump pickup got blocked & seized the engine.
If the oil light comes on, at like 2500-3500 rpm. Turn it off right away.
Wait a bit, start it U again, if it's still on, I dunno.
But if it's off, dont rev it, the pickups blocked, so its still fixable.
But that's another story. ( Drop sub frame, lift engine, remove sump, replace oil pump, clean out sump of cru & such, replace.
Our NewZealand 1800 engines are normally aspirated ( ADR ) & turbo ( Dunno ) American seem to only have turbo versions.
Lots more personal A4 experiences to come..