Thanks guys... As soon as I read your posts I checked for a clogged hole in the little grill that feeds air to the temperature sensor. Sure enough it was chuck full of caked dust. There was a hat pin laying in the console next to the seat so I used it to pick and scrape the dust from the hole.
The climate control works again! I can now remove the parka from my back seat.
Lesson Learned: Hat pins should be added to the Mercedes tool kit.
