Any ideas why there would be no air coming from the demister vents?

I have recently purchased a 1998 BMW 316i SE and all electrics etc are working as they should apart from there is no are coming through the windscreen/demister vents at all. The car has a center climate control unit to adjust the air flow/ temp settings, when the front windscreen demister button is pressed the light comes on and I can hear an increase and change of direction of air flow, but nothing comes out of the vents. All other vents/temps work fine.
I am unsure how to access these vents to look for any visable problems.
any ideas of what might cause this?
Many Thanks
Mark