1991 Chrysler New Yorker foggy Windows

Sounds like the heater core is leaking.

No, not necessarily. It may be going out the condensation drain.

You should have the system pressure tested.

The entire HVAC housing assembly has to be removed and disassembled.

The manufacturer has supplied written instructions only, no diagrams.

REMOVAL AND INSTALLATION

From inside the engine compartment, disconnect battery negative cable.
Discharge A/C refrigerant and disconnect lines at unit.
Drain radiator and disconnect heater hoses at unit Tape heater tubes to keep from leaking during removal.
Remove A/C condensate drain and disconnect vacuum lines.
Inside passenger compartment perform as follows:

Remove right upper and lower underpanel silencers.
Remove steering column cover.
Remove left underpanel silencer.

Position front seat or right front seat full rear.

Remove right cowl side trim.

Remove glove box.

Remove right instrument panel reinforcement.

Remove ash receiver.
Remove center distribution and defroster adapter ducts.
Disconnect relay module.
Disconnect blower motor wire connector.

12 Disconnect demister hoses from top of unit

For NonATC, disconnect the temperature control cable flag from the bottom of the heater- A/C unit and unclip the cable from the left side of the heat distribution duct Swing the cable out of the way to the left.
For NonATC, disconnect the vacuum lines at the unit.
For ATC, disconnect the instrument panel wiring from the rear face of the ATC unit.
Disconnect right 25way connector bracket and fuse block from panel.
From engine compartment, remove 4 unit attaching nuts.
Remove unit hanger strap lower screw, and rotate strap.
Move heater-A/C unit rearward to clear mounting studs and lower.
While pulling the lower right of instrument panel rearward:
Rotate unit while pulling from under instrument panel.

To install, reverse the preceding operation.

Sure, that is easy. Just remove the 2 heater hoses at the firewall and rig a way to connect them together without kinking them. The parts stores have connectors to do that as the hoses may be 2 different sizes. Of course you realize that you will have no heat or defrost then.