How do you replace a Freightliner fld 120 heater core?

To replace the heater core in a Freightliner FLD 120, start by disconnecting the battery and draining the coolant from the system. Remove the dashboard and any panels obstructing access to the heater core, then disconnect the hoses and mounting brackets. Carefully extract the old heater core and install the new one, ensuring all connections are secure. Reassemble everything in reverse order, refill the coolant, and check for leaks.