Might also be a badly clogged pollen filter.
I understand you DO get outside air on the lower settings of the fan, right? Just not very much. When the cabin air filter is clogged you get problems like this.
The filter should be replaced, not cleaned.
To access the filter iirc you have to remove the panel in front of the windshield, passenger side. I believe you have to move the windshield wipers to the up position and turn off the ignition to be able to access everything.
Search brought up these among others:
http://www.contour.org/forums/ultimatebb.php?ubb=get_topic;f=3;t=006690
http://www.contour.org/forums/ultimatebb.php?ubb=get_topic;f=2;t=002557
hth
Joerg
