A/C not blowing

[Limeyboy]

The a/c works fine in my 70 mach 1 and even though the centre vent connection tube is missing she used to blow lots of cold air out of the centre duct.
Yesterday I changed from heating the screen ( its nearly winter here!) to a/c and the air still came out the screen vents and not the a/c centre and side vents. I cant get it to blow anything out the floor vents either.
I took the centre vent out ( it's not fixed, no fittings) and could feel / hear something moving in the dash but the air still only seems to come out the screen vents.
I've had the dash out before to change to LED bulbs but don't want to again if possible so can anyone tell me which bit has broken, I saw a white plastic arm on the right hand side of the screen vent that looked like it could do with changing when the dash was out, could this be the problem or is it a vacuum line job?

I assume that when you turn the a/c on all the air is directed out the centre vent and the two " side eyeball vents" so it's just a matter of finding which flap isn't moving?

Thanks