Climate Control

I have a 3500 seriers pickup, year 2000. I live in California and run my A/C alot. When I turn the select knob from floor to mid-cabin, nothing happens. The A/C works, the blowers work, it just won't work at mid-cabin. I can turn the A/C off and get air to blow on the windshield and I have, on occasion, had the mid position working (1 in a hundred).

It's hot right now so if anyone has any suggestions I would appreciate any help!

Yours truly,
114 degrees yesterday