I've seen where it wasn't the heater core but one of 4 internal pipe o-rings. Remove the left lower panel, then the plastic cover to the right of your gas pedal. It's likely to be a bad pipe o-ring (2 pipes go between the heater core and firewall connector), more likely that than a heater core and those pipes can be removed without removing the dash as I've done it.

The bad part is that if enough coolant has already saturated the padding under the carpet, it's a nightmare trying to get rid of the mini-lake of coolant. We had a car last summer that had that happen to it and there was so much coolant under there that it required all new carpeting and padding as it was ruined. Fortunately for you it's still warranty.