1996 Ford Ranger heater core

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.
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.