Dear customer, a water inlet can come from different areas, it is good to start with the channels, but it is clear that your filtration goes beyond these.
Water filtration as long as it does not affect the luggage compartment, but only the floor area of the rear seats focuses only on a leak in that area of the roof or windows.
Dear customer, I am going to recommend you to do the following:
Remove the seats from the back of the vehicle.
Ask a friend for help, he will located inside the vehicle with the roof and the windows closed, you will use a hose and you will try to lift the vehicle from the back to tilt the vehicle, if it is already inclined there is no problem, you will start to pour the water in the back of the roof (As the vehicle is tilted is very likely to be only here).
The person inside the vehicle will observe the leak or the space where it leaks.
If you notice that the filtration is in this area, you will see an internal rubber, it is going to have to be replaced, you have to dismantle it by the trunk to replace it, anyway this rubber is not expensive to replace it and a technician can give you a hand
There is a possibility that the filtration is not from the back but from a side, so you will use the hose to pour the water through all the slots in the window, where there is leakage, you will replace the rubber on the window glass or see if it need to be stick again. What happens is that these rubbers with the sun begin to crack and it is here where the leaks happen.
To replace the rubbers of the window itself is a bit more complicated because you have to disassemble the glass to put the new rubbers and use a special sealant .