If you have the ambient air sensor you'll see it on the top of the dash, right smack dab in the middle: a small square grate of sorts, same color as the dash.

I believe that this is a conduit for ambient cabin air which in turn "tells" the climate control whether ambient air temp is higher or lower than the desired temperature, then the climate control knows whether to make hot or cold air...

Over time, the foam hose that Ken is mentioning can deteriorate and will need replacing. A cheap and easy job to do, really.