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