There is likely to be damp inside the car, under the carpets. Check, then find the leak. It's probably coming in through the pollen filter because the bulkhead vent wells are blocked, leaving rainwater coming in though the air intake nowhere to go except into the car.

Even if the drains are not blocked, the pollen filter could be sodden, preventing fresh air from entering the car. The cabin vents that let air out through the sides of the load area could also be obstructed. In the meantime, probably better to demist with a window open.