i realised that mine's also been suffering from this (thought w/screen setting was ok, but i do get cold feet - doesn't seem to switch to footwell setting)
but i got misted up screen today & realised airflow was pathetic (compared to airflow from centre vents on dash)
i popped the side cover off the transmission tunnel in passenger footwell to check things out (w/shop manual said the airflow direction knob controls a bowden cable which connects down here)
sure enough - problem. centre wire of bowden cable was still connected to the airflow direction 'lever' on side of air distribution box...but the sheath of the cable had snapped free from its plastic clip mounting. so turning the knob on dash caused the sheath to move around, but not the inner cable or airflow control lever
i couldn't really get my hand in there to do a proper fix (screw to clamp the sheath is visible, but behind the metal & vinyl dashboard panel...) so i bodged it up with 3 or 4 strips of gaffer tape, sticking the sheath to the side of the air distribution unit, being careful not to stick the airflow control level down too!!
the centre cable now moves in/out with the control knob on dash, so i should have clear screen/warm feet tomorrow
don't know how long it will last tho...