How do you replace the heater core on a 98 Mazda 626?

To replace the heater core on a 1998 Mazda 626, first disconnect the negative battery terminal and drain the coolant. Remove the dashboard components and center console to access the heater core housing, then disconnect the heater core hoses from the engine bay. Unbolt and remove the heater core from its housing, replace it with the new unit, and reassemble everything in reverse order. Finally, refill the cooling system and check for leaks before reconnecting the battery.