and how can I replace it?
where does cabin air filter located? on 1990 Audi Coupe Quattro
the first audi in usa to use a cabin filter was the 96 a4, you don't have one.
I see one listed (with picture) on www.rockauto.com but I do not see instructions, location or price of parts in any information I look up. I don't think it actually has one. Some cars have them behind the glove box but if there is actually one fitted in your car it may be in the engine compartment under the wiper arm panel.