I had a similar problem a few months back. I would only get air from the defrosters no matter the setting. In remembered reading somewhere that vacuum losses can cause such a problem. After a few hours of searching I found a vacume leak in a hose running down the firewall. The tube had been resting on the manifold and burned a pin hole in it. Once I replace the section of tube (about $1.50) my switch has been working correctly ever since.
