heater blows cold and only throught defroster

I've just bought a 2000, 300M. The problem I have is the heater blows ONLY cold air and ONLY thru the defroster. I have gone to the local salvage yard and replaced the control head unit TWICE and I haven't fixed the problem yet. Does any one have any experience here that can help me?
PS: No problem that I can see with the thermostat the temp gauge goes up 1/2 way @180/190.

I had a sightly different problem but this solution worked for me, if you have auto air:

1. Start the engine but do not move the vehicle
2. Set the Temp control to 75 degrees
3. Press and hold the Floor, Mix (vent/defrost) and Defrost buttons at the same time. The floor is a bottom button and the others are upper
4. The control head will begin to blink and enter a diagnostic mode
5. Release the Floor, Mix and Defrost buttons

Once the unit has gone through its diagnostics it will either return to its normal mode (with all air doors calibrated) or it will return one or more of 16 different error codes between 23 and 51. The ATC control can only show one diagnostic trouble code at a time. Under certain circumstances, more than one diagnostic trouble code could be in the membory. To scroll through any additional diagnostic trouble codes, press the Panel Mode button on the ATC control. Any other button or turning of a knob will end the diagnostic test without showing the rest of the trouble codes.

I believe you have a blend door problem. You need a new one.