My 924 is unusual in that I can turn my heater fan off completely. In the fully off position(no fan running), there is also a cover which closes over the heater venturi (which is inside the black plastic housing, sitting on the firewall). It looks like a factory fit feature, never been altered from new. It's a different layout to my "parts" 924 which is only 6 months newer.
Also, at some stage in it's life, someone has fitted a vinyl cover over the dashboard top to hide the cracks. They have fitted vents to the top to facilitate windscreen demisting, but these vents don't quite match the original vents, so that I don't get full air flow through them. The result is a screen which is much more difficult to clear when it is misted up.
The screen also has a tendency to mist up on the outside when driving, I think this is to do with a low pressure area at the base of the screen where it meets the bonnet. This misting clears when I operate the wipers.
Never thought that I could write so much about misty windscreens
David