Water is getting into the car. I think it could be the air conditioning.
Hello. During it’s operation, the air conditioning system will come across a large amount of condensation. This condensation gathers around the evaporator and runs off through a drain. You may see water dripping from the car on a hot summer day while the air conditioning is running. The drain leads to the engine bay in most cases and dirt may accumulate in the drain tube. If the condensation backs up, water can leak into the car.
Another note, the heater core which is located inside the cabin as well is a components used to heat the passenger compartment. Inside this device runs hot engine coolant and when air is blowing across, the air picks up the heat and heats the cabin. A leak here is a cooling system leak and can also spill engine coolant and water inside the vehicle. If you need further assistance, I recommend you have a certified technician, like one from YourMechanic, inspect heating/AC system and suggest an appropriate repair.