I had a similar problem a few years ago and it had to do with the mechanism (actuator??) that opens/closes the vents. The head-unit was fine, but the air only came out of the floor. I had it fixed at the mechanic's shop, so I can't tell you exactly where the problem was...only that it was in that mechanism.
Has worked fine ever since...but, now my blower fan is noisy...
Good luck
