Unfortunately, this generally means your Heater Core is leaking & will need to be replaced. You probably also have a film on the windshield that is anti-freeze based due to the steam coming up through the vents. You'll want to fix this before it causes engine problems as the cooling system must maintain pressure or more damage to other components will occur. The part is less than 40.00 at most parts stores. Good Luck.

Inside the car?

you possibly have a heater core leak....is the pass. floor mat wet or damp....most times when a heater core is leaking, you can actually taste it in the air....it's actually sweet.

have it fixed, or bypass the core itself if it gets really bad.

hope this helps...