Sounds like you've got two issues happening here.

1st in that the diverter motor that sends the air either to the windshied/vents/floor is not functioning properly. I've heard that the resistor bank that controls this may be the cuprit. I've had this same problem for years on my 92. I have to leave the button depressed on floor/windshield for a few days before it decides to kick in. Then I leave it alone for the winter.

2nd your A/C might not be working properly. If you put it on recirculate and run the A/C for a bit even if the air is going to the floor it will eventually get rid of the fog on the inside of the windshield.