2006 Citroen Other heating won't clear windsceen

Hi,

i have a citreon relay.

the heating system is working, but it is not clearing the windsceen.

when i turn it to feet or side vents, hot air comes blasting out.

however when i turn it on clearing the windsceen no air comes out.

i imagine that some inturenal direction flap for directing the air has broken. is there anyway of fixing it my self!?

cheers