Not Heater Core
No it is not leaking anything. We have gone out and turned the defrost, all the way to hot to defrost the windows then we turn it down, once we are driving for a while all windows will start to fog up. We recently had to replace an evaporator core on our AC unit. I think but i am not sure that our switch for the air circulation to bring air in from outside is not hooked up, from when the dash was apart. I will try the AC method to see if it helps....Thanks.