Ol'Timer - Do you mean pouring water onto the rear passenger door window to see if water is getting in at the bottom of the glass there - and then seeping down
inside the door?
And sorry to keep going over old ground - but how likely is it to be the Pollen Filter, given that the front footwells are dry? - it's only the back ones that are wet....
If you open the door and dry under the door card, then pour water along the bottom of the glass, some of it will run down into the door cavity (perfectly normal.) If all is well it will run out of the drain holes at the bottom of the door.
It is possible for the drain holes to become clogged with wax so it is worth checking they are clear. If the seal between the ancillary carrier and the door has broken down you may see water seeping out frum under the door card and it will be worth a closer inspection.
If the pollen filter shield is not fitted correctly water will drip down from under the dashboard on the passenger side and be quite obvious. This happened to me when I had a new windscreen fitted.
Out of interest as you've got the seats and carpets up at the moment can you see if the centre of the rear footwell is the lowest part of the floor as this is where the water will settle.
On my Leon all the doors leaked, the bulkhead leaked through a badly sealed seam and the pollen filter leaked. I didn't realise anything was wrong until a terrible smell appeared.