Hi, rear footwell is filling up with water behind

Have a look at this
http://www.weirdlittlebiscuit.com/passat/leak/

I have the same problem at the moment as well

It's probably coming from the sunroof drains being clogged.

Open the front door, look closely, you will see a rubber tube sticking out 1 inch. Cut off the tips so the tube is wide open.

These leaks are tough to trace sometimes, but with a garden hose or buckets of water, some patience, you should be able to determine where the leak is. It would be best to do this when everything is dry, including the footwell area. Lay down some paper towels, and direct the water at a spot on the door , keeping it in one specific area for a while, while a helper is inside with a great flashlight, looking for evidence of leakage. It is most likely the rubber seal above the door. Carefully check the seal for damage, or a 'pinched' area that allows the water to get by. Make sure the surface that the door seal makes contact with is clean, sometimes a lot of junk (from leaves, pine needles, dirt, grime) builds up, blocking the flow of water away from the door gasket (seal), and into the vehicle. Hope this helps.

it can also be the leak from the rear window seal, the one aboove the trunk if you know what i mean