Is your climate control working ok? Do you have window protection/treatments on? Have you checked your door seals?
Also check;
- the pollen filter isn’t saturated with water
- if the car has a sunroof, the drain pipes aren’t blocked and letting water into the cabin
- the carpets are dry (if they’re wet, probably a sign of leaking seals, or blocked pollen filter)
- water isn’t getting into the car around the door speakers, as the speaker seals have been known to fail
- there isn’t water in the spare wheel well in the boot. If there is, possible causes might be a defective light cluster seal, a perished drainage tailgate grommet, the rubber concertina sleeves that carry electrical wiring and washer fluid pipe into the tailgate might be damaged. Also check water isn’t getting into the boot around where the tailgate hinges are attached to the car body shell (a problem on early mk7’s).
- any damp / wet outdoor clothing and umbrellas have been removed from the car.