whats causing my heater to default to windscreen only?

i have a 1996 ford mondeo ghia x turbo diesel and for some reason the heater stays on the windscreen setting no matter where i turn the adjustment dial to (i.e footwell, dash vents etc), the heater works on all speed and temperature settings, it just blows it on the screen and no where else. i have checked my coolant level just to be on the safe sideand its fine. as the weather is kinda freezing at the mo, and im getting tired of poached eyeballs! lol i wondered if you knew what was causing this and how i fix it?