Citroen Xsara 1.9 D of 1997 Spraws problems

Good evening everyone. I have a 1997 Citroen Xsara. The air conditioning works fine; I've replaced the cabin air compressor. I have a problem:
the air isn't blowing from the upper part of the dashboard. I've checked the small motor and the gearbox that controls the hot and cold air, and the vents are working fine too (I've removed the dashboard). Is there another motor or vent I'm missing, or is there a vent that I haven't noticed that's stuck closed?
I would greatly appreciate your opinion.
Thank you very much.