If the vehicle came from the factory with air conditioning, it has a drain pan under the evaporator in the dash. Even if the heater core was leaking, the coolant would collect in that pan and drip onto the ground, not the floor. Look under the hood on the passenger side of the firewall for a 4" long rubber hose hanging down. Squeeze the end of it to open it up and dislodge the blockage. If nothing comes out, squeeze the wire ring that holds it on, remove the hose, then carefully stick a pencil into the spout to clear any obstruction.