Heater core

If it's leaking, it gets replaced. Heater cores aren't repaired. Replacing one is not a do-it-yourselfer job. The air conditioning system must be discharged with special equipment, then the dash must be removed to get to the heater box. If you have any thoughts of doing this yourself, get a copy of the manufacturer's service manual and read through the many pages of instructions to see if you really want to do this.

Replace the antifreeze when the job is done. Acids build up in it over time. New antifreeze has anti-corrosion additives but those wear out in about two years, then that acid is the most common cause of leaking heating cores and radiators on GM vehicles.