And it's only the carpet we are talking about.
As you will have read from the first reply to your question the foam Seat used goes brittle over time and breaks away. This leads to water getting in through the doors, the drain holes get blocked, the bottom of the door fills and spills past the bad foam under the door card into the front or back footwells.
In the scuttle bay the cabin filter and wiring loom entry are also known leak areas.
In my first Leon I cut a square out and got the water out, the second Leon I used the syringe method. I had a pool of water in the rear wells and got rid of it all as I previously described. Neither car has since leaked 5 years on.
As it's the lower section of the backing plate you need to worry about the job can be done without total removal and having to worry about the carrier and glass. I got a couple of longer bolts to hold it in place but allow more wiggle room while I worked.