Hi. I had the same problem with my 200 last year and drove myself crazy trying to locate the source but did find it.....eventually!
Open the rear door, look down! There is a plastic strip that runs across the vehicle on the floor just inside the lip. Pull this strip up and there is a narrow tray underneath. Follow this tray and, about 6" in from the C Pillar on both sides there is a weld seam. Rain was seeping in through this seam, pooling in the tray and slopping over onto the rear floor pan. I tried several things but in the end (having replaced the floorpan with a galvanised sheet supported by box girder!) I simply packed this tray (and especially the area around the seams) with black guttering mastic....stickly as hell, but it has done the job!
Your window seals are certainly a possibility (pull away the plastic covers on the C Pillers and you should be able to see any obvious stains where the water is running), if so, try washing your landy with Myrh car wax/wash: A friend of mine who takes landys apart for a living swears by the 'sealing' properties it.
Rich