haha. I have the same problem, but it only comes out from the vents on the floor and the two outer vents on the dash.
if I change it to defrost that will come on too, but with the floor and the outer vents.
I asked a similar question, and was told the problem could be the climate control unit itself
I was also told I could check the continuity on the unit to see if its defected.
If you figure it out post it up.