How do you replace heater core on 99 Cadillac sts?

To replace the heater core on a 1999 Cadillac STS, first, disconnect the battery and drain the coolant. Remove the dashboard and center console to access the heater core, which is located behind the HVAC unit. Disconnect the heater hoses and any retaining clips, then carefully pull out the old heater core and replace it with the new one. Reassemble the dashboard, reconnect the hoses, and refill the coolant before testing the system.