I would have to agree with Bob's diagnosis. Does the smell of the air out of the vents smell sweet? If so, your heater core has given up the ghost. If the heater box is anything like a Mustang's then you are in a long DIY project or a heafty shop bill.
The only other things I can think of are a water logged air cabin filter or a clogged evaporater drain line that is not allowing the coil condensation to drain out of the heater box. HTH.
