99 Blazer A/C controls

I just bought this Blazer recently and now I'm finding all the problems.

When I bought it, the controls all worked. Now, the flow never changes from the vents no matter what position you put the controls in... (A/c, defrost, heat) the air always comes out the dash vents. The heat is hot and the air is cold like it should be. Does anyone have any ideas?

Thanks form a newbe...

I had the same problem, and it was the selector switch itself. A new one is 125$ at a dealership. So I tore mine apart and a rubber diaphram was leaking air, so i put a washer behind it to give it support. Does it sound like hissing air under the dash? If it does that is probably it.

mine is definitely hissing from the rubber diaphram. where did put the washer behind? i can't see where a washer would give any support to the rubber diaphram.