Around three years ago I had a very wet carpet on the driver's side under the seat. Checked the filter and sponge and it was pretty well clear. Water was still getting into the cabin rather than going down the drain tube. I tried using the trombone brush that had worked so well on my NA and NC but no way could I get it through on the ND. There are too many twists and turns in the tube and top and bottom assemblies.
It would seem that the filter assembly will stop some detritus getting through but anything smaller than a large twig or dead sparrow will find its way into the tube and cause a blockage somewhere near the bottom.
At that time the car was still under warranty and my dealer cleared it for me with no charge. I believe their final solution was to use an air line to blow out the blockage.
So fast forward to now and I found I had something resembling a pond under the passenger seat. Out came the wet and dry vacuum and sucked out four litres of water! If you haven't got one I can recommend the Karcher MV2. Filter checked and all clear so rather than pay the dealer a big wad of cash I thought I'll see what I can do myself. Again I couldn't get the trombone brush all the way through so more investigation required.
Boycie's instructions above are spot on for the filters but if you need to go further, read on.
These are the trim pieces and rear carpet that I removed.
Might look like a lot of dismantling but it's six trim clips to remove and then everything just pulls out. Put the trim clips in a container and put the lid on before you kick it and send everything flying into the flower bed!
Now clean the filters as described by Boycie.
So if the tubes are still blocked you need to clear them by using a trombone brush from the bottom. There doesn't appear to be any sort of flap on the bottom of the tubes on the ND. After pushing the trombone brush a couple of inches in the water and gunge started flowing out. I few seconds of pushing and pulling cleared them completely. This is all well and good if you have a ramp at hand for access underneath but if not here's what to do.
Look at the panel behind the seat and you will see this access panel. The cover just clips on and off.
Pull the cover off and inside you will see the bottom of the tube and the assembly that clips into the bodywork to take the drained water to the outside. Twist and pull the assembly and it will pull out from the bodywork like this.
Now it's easy to get access with the trombone brush to clear the blockage.
All done so clip back the tube and replace all the trim. Pour a little bit of water on the rear of the hood and you should see it all flooding out of the drain under the car.
After I had done the passenger side where the problem was I moved round to the driver's side. The whole procedure including stripping the trim, cleaning the filter, clearing the tube and putting everything back together took me 20 minutes.