Heating/Colling issue with '93 300SE

I have a '93 300SE which has a wierd problem with it's a/c. When I turn on the system with temp set either to low, or extreamly lower
than the outsied temp I do not get any cold air. This started as just starting to intermitently push hot air (all hot not coolr at all) through the outer vents, and now has progressed to hot air only through all vents. I have checked the compressor, and freon level, and all that is correct. Even with winter coming on the heater on full will be causing us to sweat.
I thought it might be that the vent that runs air across the heater core is stuck open, but do not know where it is located.
Esentially I have two questions:
1) Knowing that the compressor is fine, and the freon level is correct what would be the best way tyo diagnose this issue?
2) Where can I get a descent service manual for this vehicle. Most of my experience is with American autos, 4-wheelers and Motorcycles so this beast is a little different.
Thanks for any help you can provide.
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