the evaporator case can hold a lot of antifreeze before it starts to get the floor wet. i smelled the moldy antifreeze smell for a few months before it started making my windows fog up with the heat on. so i would say that your heater core probably has pin holes in it. do you ever hear a gurgling noise from the passenger side of the dash?

 

+1 im not a heating expert but ive seen many thrads on your type of situation, and the results are bad heater core