If the fan is working, I'm guessing you have a mode door or actuator problem within the Heating, Ventilation and Air Conditioning (HVAC) air handling system. The HVAC system is uses small electric motor actuators to control plastic doors.
- There is a blend door for temperature (full cool, full heat or somewhere in between)
- There is a recirculation door for outside air (useful in the summer to improve air conditioning performance)
- There is one (or more) mode doors that determine where the air exits the system (defroster, panel-vents, or floor)
I'm pretty sure your mode door controller is having troubles. Its possible the motor is burnt out. It's also possible that the plastic door is broken or cracked inside the HVAC system.
Actually you can perform a test for this yourself. Sit in the car in the front passenger seat. Turn on the ignition power (Do NOT start the car!) . Turn off the radio. Place blower controller on its lowest setting. Now place the system in different modes (Floor/Heating, Panel/Ventilation, Defrost). It may help to open the glove box. You are going to want to listen carefully to what the system is doing. If you can hear absolutely nothing, that's probably a dead actuator motor. Pretty easy fix. Parts are $40 or so. If however you hear the electric motor turning, accompanied with a bunch of clicks, that indicates you may stripped plastic gears inside the motor actuator, or even that you have a broken plastic door. The motor actuator has a keyed shaft and that key is slipping. Do note, if that door is broken, this is a big repair job, involving removing the instrument panel, replacing the HVAC system, etc.
Do note.. If you are stuck in floor mode, this has to be fixed. ITS DANGEROUS to be driving this car in the winter, particularly if it is raining or snowing. Your windshield can fog up and you won't be able to see the road to drive safely. No amount of driving will help, in fact continued driving will continue to fog the windshield in certain weather conditions.