99 sub hard to defrost

I am having trouble keeping my side windows from freezing up. They start out clear and then freeze up as the car warms up. The front window will stay clear as long as I keep the fan on the second position. I was thinking my heater core might have a pin hole leak and was misting the windows with glycol, but the windows aren't slimy.The overflow was almost empty the other day and it seems to be doing better after I added coolant. I haven't had a problem with it not blowing it warm air. Any ideas?