How do you replace heater core on 2003 Audi a4 1.8t?

To replace the heater core on a 2003 Audi A4 1.8T, start by disconnecting the battery and draining the coolant. Remove the dashboard and center console to access the HVAC unit, which may require removing various trim pieces and screws. Once you have access, disconnect the heater core hoses from the engine bay, then unbolt and remove the heater core from the HVAC unit. Install the new heater core, reassemble the HVAC unit, and reconnect everything in reverse order, ensuring to refill the coolant and bleed the system.