1991 Mercury Tracer trying to get a picture showing how to

Disconnect the negative battery cable. If equipped, disable the SRS system.
Drain the cooling system.
Disconnect the heater hoses at the firewall.
Remove the instrument panel.
Disconnect the mode selector and temperature control cables from the cams and retaining clips.

Remove the necessary defroster duct screws and loosen the capscrew that secures the heater-to-blower clamp.
Remove the three heater unit mounting nuts and disconnect the antenna lead from the retaining clip.
Remove the heater unit.
Remove the insulator and the four brace capscrews.
Remove the brace.
Remove the heater core from the heater unit.

To install:

Install the heater core into the heater unit.
Install the brace.
Install the brace capscrews and the insulator.
Place the heater unit into position, then attach the defroster and floor ducting.
Install the heater unit mounting nuts.
Tighten the heater-to-blower clamp capscrew and install the defroster duct screws.
Connect the antenna lead to the retaining clip.
Install the instrument panel.
Connect and adjust the mode selector and temperature control cables.
Connect the heater hoses at the firewall.
Fill the cooling system and connect the negative battery cable.
Start the engine and check for leaks.
Check the coolant level and fill as necessary.