This is a big job. The heater core is located in the heater case inside the dashboard. The entire instrument panel must be removed to service the heater core. It is located on the passenger side, behind the dash, inside its enclosure/plenum box.
The instructions for the heater core removal are below.
I tried to include the instructions for the instrument panel removal, but the pictures made the file too large.
Here is the text for the instrument panel (dashboard) removal:
CAUTION: Electronic modules are sensitive to static electrical charges. If exposed to these charges, damage may result.
WARNING: To avoid accidental deployment and possible injury, the backup power supply must be depleted before repairing or replacing any air bag system components. To deplete the backup power supply energy, disconnect the battery to starter relay cable and wait one minute.
WARNING: Always wear safety glasses when removing and handling an air bag.
WARNING: Carry a live air bag with the bag and trim cover pointed away from your body. An accidental deployment will then deploy with minimal chance of injury.
WARNING: Place a live air bag on a bench or other surface with the trim cover up.
NOTE: Position the carpet aside.
NOTE: Two technicians are required to perform this step.
NOTE: Two bullet connectors secure the instrument panel to the bulkhead.
NOTE: To prevent any squeaks or rattles, install and tighten the upper instrument panel floor brace bolt first, and then tighten the floor brace bolts and nut.
NOTE: Reposition the carpet after this step is performed.
WARNING: Never probe the connectors on the air bags. Doing so may result in air bag deployment, which could result in personal injury.
WARNING: Always wear safety glasses when repairing an air bag vehicle and when handling an air bag.
WARNING: Carry a live air bag with the bag and trim cover pointed away from your body. An accidental deployment will then deploy with a minimal chance of injury.
NOTE: When the battery (10655) is disconnected and reconnected, some abnormal drive symptoms may occur while the vehicle relearns its adaptive strategy. The vehicle may need to be driven 16 km (10 miles) or more to relearn the strategy.