Water in the trunk

Good day together, The C1 of a friend (Bj 2009) is the third car (including mine) of this type, which I know, in which it is always wet in the trunk. Newspapers, which are designed to help with drying, are after a short time (depending on the weather) through and through wet. Sometimes the water is finger-high in the spare tire compartment. With me it is not quite so bad, but something in the trunk also can not be done with me. At the seal of the trunk window are no mistakes to fin Unfortunately, you can’t see where the water comes in. As I know this state of several C1, it is obvious that there is a general problem (constructional error) that is the cause. Does anyone know this problem? Does anyone have a tip on where the water might come from? Thank you!