Heater core

Here are the directions I could find for you:

Disconnect the negative battery cable. Allow the engine to cool down. Drain the cooling system to a level below the heater core fittings on the firewall.

Disconnect the heater hoses from the heater core tubes and plug hoses.
Remove the five nuts that secure the blower motor housing to the firewall.
Pull the housing away from the firewall. Disconnect any wire harness plugs which inhibit the housing.
Remove the remaining nuts which secure the heater air plenum (inside the passenger compartment) to the firewall.
In the passenger compartment, remove the instrument panel. Refer to Body & Trim.
Pull the heater air plenum rearward from the firewall and remove it from the vehicle.
Remove the heater core from the heater air plenum.
Inspect all air seals on the components and firewall and replace any that are damaged.

To install:

Install the heater core into the air plenum chamber.
Position the heater air plenum against the firewall from the passenger compartment.
Under the hood, install the plenum retaining nuts.
Install the blower motor housing to the firewall and install the attaching nuts.
Connect the heater hoses to the heater core tubes at the dash panel in the engine compartment. Do not over-tighten the hose clamps.
Install the instrument panel.
Check the coolant level and add coolant as required. Connect the negative battery cable.
Start the engine and check the system for coolant leaks.