Interior warmer not working

do you mean you need to change the heater core? (German "Wärmetauscher"?)

For that, you need to dismantle the heater assembly located in the blower compartment ("Gebläsekasten") behind the engine. That´s NOT a very fun thing to do.

Look here for a description in German on how to dismantle everything.
This is also described in the "Reparaturleifaden" (official factory repair manual) that you may download here

OK, it seems that your BLOWER MOTOR is shot.

Buy a new one @Audi. Part number. Part number is 443959101. Cost in Germany 106 Euros.

Or, you can repair this for mext to no money (it`s just the carbon brushes that need to be replaced, and these cost a few cents)
There`s a howto in the FAQ, and quite a few experiences in the German Forum (this one for example)
I would strongly advise to refrain from putting a used part in there, as it is a five-hour work to rip everything apart because this damn thing is a PITA to get at and with used parts you might have to do it again very soon.... See attached pic for details.

Hi,

forget my above posting! This is vaild ONLY for V8 pre-facelift (with the "old" AC controls).
When you have a <'92+ V8 with the NEW climate control, the fan from a Type44 Audi will NOT fit! I didn´t look this up correctly !

The part number for the "new" fan is 4A0959101A. It is significantly more expensive than the old one - price in Germany is 154,28 Euros.
see attachment.... the fan is No. 40...

sorry for the confusion!