I think that one of the notorious clips may have come undone under the passenger side dash - there are two there: the upper one for the heat/cold mix; and the loewr one I think that handles the direction of the airflow to the windscreen/footwell. If the upper one fails (which is apparently the common case then all you get is hot air in the cabin) and I've read somewhere that if the lower clip breaks then you don't get airflow to the windscreen and only to the feet.
try this page on clarks-garage.com:
http://www.clarks-garage.com/shop-manual/hvac-01.htmI'm no pro but I hope that helps you trouble shoot.
Right now I'm trying to get hot air IN to the cabin by making these undoing these clips... LOL