How can I fix my car air conditioner?

I have a 97 Mercury Mountaineer. This past winter, whenever I turned my air conditioner/vent/heater on it blew hot air, no matter where the temp control was set. I just took it in and the mechanic had the air conditioner charged and changed the switch that controls what vents the air is going to come out of i.e. floor, defrost, outside vent etc. After that I could get cold air on the max air setting but all the other settings still blow hot air. Even the ones that would normally blow cold air.

What should we check now. The Ford dealership said they think it is the entire air/vent/heat control system and it would cost $600.00. Is there a name for this part? I would like to check the junk yards if I do have to get this part.

Thanks for any advice you can give me.