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It appears your at the point where you need to remove a few nuts that secure the chamber, then remove the chamber, and then you unscrew and remove the heater core cover:
Remove the instrument panel.
Remove the evaporator core housing.
Drain the radiator.
Disconnect the heater hoses from the heater core.
Remove the nuts and the plenum chamber.
Remove the screws and remove the heater core cover.
Remove the heater core.
Installation
NOTE: Be sure to install a new oval foam seal around the heater core inlet and outlet tubes.
To install, reverse the removal procedure.
Lubricate the hoses and fittings with Ford Rubber Suspension Insulator Lube E25Y-19553-A or equivalent meeting Ford specification ESF-M99B112-A.
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