Front windscreen heater vents (2 don't work)

[j_s]

Hi guys

Out of the 4 vents that are on top of the dash to demist the front windscreen only the 2 middle ones work, the other 2 don't have anything coming out of them. When at the scrapyard on Sunday me and Rubjonny looked in the dash of another mk2 and only 1 visible pipe goes to the center of the dash and the 4 vents are fed from that.

What could it be 

Obviously with winter in it's way and it's already getting damp now i'd like to fix it 

[Gambit]

the heater direction mechanism probably isnt opening the slot fully

[j_s]

Thanks i'll take a look at that