I had a bad leak in the back seat of my 2002 Polo. The seat and floor were soaking we. I thought at first the window was left open during heavy rain or a wash. I doscovered two possible reasons for the leak:-
(1) in the boot on the left side if you pull back the side wall upholstery you will see the rear left seat, seatbelt roller. just below that you will see a block of electrical connectors and a 10mm black pipe with a male/femele connector. That thin black pipe is the rear window washer water pipe. That pipe was disconnected at the join. This can happen I believe during freezing conditions. I reconnected the pipe and put some electrical insulation tape around the area where the male/female clips together. I am now waiting for back seat to dry and see if that has solved the proble. The floor mat in the rear was also wet. If you cannot find the connector, have someone operate the rear washer and firstly if no water squirts on the window, the pipe is possibly disconnected. While the helper is squirting, examine the area in the boot on the left side behind the side-cloth.
(2) I removed the rear left tailight cluster, which is held in with a single black size 10mm plastic nut attached to a tie to prevent it from falling into the wheel arch when removed. Having removed the nut, lift the tail-light cluster up slightly and pull back and the whole unit comes out. At the point where the cluster meets the metal of the car, there is a black rubber gasket to prevent water from getting in around the tail light. The black rubber gasket was damaged and not making a good seal. I discovered this when I reconnected the washer pipe, when water that ran off the window was "*******" in at the back of the light cluster. I put some bathroom silicone around the area where the gasket was damaged and then ordered a new gasket. The new gasket comes in a kit of two big gaskets and two smaller ones, all associated with the rear light cluster. The kit cost me 9 Euro per side. That is 9 Euro for the left side and 9 for the right, if you need both. This is also how you change a rear light or indicator bulb. Hope this helps someone. It's a cheap fix to a terrible problem with water leaks, which then lead to electrical shorts as there are block connectors in the area of the back lights.