Do you know if it happens when you wash the car or when you drive on wet roads, my old 200 used to leak upwards :S from the hole in the wheel well were the wheel bolted into when i drove along ( took ages to find it) your best bet is to take as much interior trim out as poss (start with the boot its easier) and spray sections vigorously with a hose to try to find the leak. Apart from that sorry mate i cant help you
Mine did this and the stealer said it was the rear lights not seated correctly.