A common entrance point is the pollen filter because after it is replaced it is very difficult to put the waterproof cap back on securely without damaging it. But that would mean a wet front passenger carpet as well as wet rear footwell. If the front passenger footwell is dry, then the water is coming in through the rear door because the waterproof membrane between the door structure and the door trim has perforated. Very common on ageing VAG cars. I used to replace them with builder's blue DPC sheeting, cut to size.