I've been there with the heater core cracking during my christmas holidays. Oh the fun times of tearing apart a dash in -30 weather, with breaks every 5 minutes to defrost the hands. Not to scare you though as usually there is advanced warning of a heater core leak. You'll usually smell something fishy and there would be a greasy film on the windows above the defrost vents. If you don't see either of those, I'd just say it's melting snow within the car.
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