Why is there water under the car floor mat?

[VonBenjamin]

Reasons for water under the car floor mat: Leaking drainage plugs at the bottom of the cabin, leaking sunroof drainage pipes, leaking doors, leaking coolant from the heater core, or damaged/improperly installed air conditioning drainage pipes. Other possibilities include: 1. Sunroof leakage or poor sealing, requiring sunroof inspection; 2. Water from car washing that wasn't completely cleaned out; 3. Corrosion or leakage in the car's small radiator (used for cabin heating). If there's significant water accumulation under the rear seat floor mat, it's necessary to remove the car seats, floor mats, carpets, and floor sponges. After cleaning the accumulated water, replace any damaged control units and check for water-damaged wiring to prevent subsequent short circuits.