How do I remove the AC housing from the firewall

Remove heater A/C unit as follows:

Remove center air distribution duct.
Temporarily tape wiring harness aside.
Remove antenna cable from retaining clip at right end of heater A/C unit.
Disconnect blower motor lead wire, then remove terminal insulator retainer from heater A/C unit support brace.
Disconnect demister hoses from tee at top of heater A/C unit.
Disconnect vacuum hose harness connector, then the vacuum feed line.
Disconnect temperature control cable flag retainer, then remove self-adjusting clip from blend air door crank.
Drain engine cooling system, then disconnect refrigerant lines and remove expansion valve. Plug all openings to prevent contamination of A/C system.
Disconnect heater hoses, then plug heater core inlet and outlet openings.
Remove A/C condenser drainage tube.
Remove four heater A/C unit attaching nuts from rear engine compartment dash panel.
Remove heater A/C unit support brace attaching screw, then swing brace to the left.
Pull heater A/C unit rearward until attaching studs clear the dashboard, then allow unit to drop down. Rotate unit in such a way so the attaching studs are facing downward, and remove unit.

You need to look around as they are there but are covered over with accessories.

Use a bright light to assist you.

Roy