The car has the 200 6cyl., have had this same problem with a Fairmont before and never figured it out. After replacing the heater core and thermostat, you have to put your hand right over the vent to feel any heat. You can drive it all day and it blows cool air. All vacuum lines to the switch are good , out of ideas. Thanks for any help.

 

Some info first please!
Do both heater hoses get hot to the touch when the engine has warmed up?
If so, there is a door on the fresh air intake for the heater. Is this door stuck in the open position causing the unit to pull in all cold air and not allowing the air to go through the heater core? You may have to remove the glove box to check these things out.
Its a good possibility one of the vacuum servos (diaphram) is leaking enough so that your heat is going somewhere else. Where is the function setting set when the air comes out the vent?

 

I checked everything out , both heater hoses are hot. The door opens and closes when moving the switch. Did find a vac connection that was open and nothing going to it. Also discovered that someone has removed the whole blower unit box at some point under the dash , a few bolts are missing. But I have another complete unit out of a Fairmont drag car if I need parts. This looks like it's going to be 'fun', will let you know what I find.