you will need to remove the dashboard and drop the steering colunm to replace the heater core,,this is a pretty big job
this may help you
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REMOVAL1. Discharge the A/C system. ".2. Disconnect the two evaporator core refrigerant lines from the engine compartment side.Cap the refrigerant lines to prevent moisture from entering the system.3. Remove the glove box and mating trim.
4. Disconnect the thermal amp. connector.5. Remove the cooling unit.6. Separate the cooling unit case, and remove the evaporator.INSTALLATIONInstallation is the reverse order of removal.
Recharge the A/C system.Inspect system for refrigerant leaks. Refer to "Checking Refrigerant Leaks".