The HVAC box holds your heater core, your AC Evaporator, and your fan.
It has a drain in it so when water runs down into it from rain, it has a place to drain.
If the box collects an unusual amount of dirt, leaves, etc, it will clog.
When it clogs, water has no place to drain, so when it fills,
it will spill over onto the floor inside the car.
