You need to find the source of the water entry, you could tape paper toweling right round the inside of the car, about 6 inches above the floor, wait until it rains, where the paper becomes wet first then that will be the source, that’s the easiest way to locate the approximate location of the leak.
If its a W107 its no new news. What the normal problem is the plenum chamber around your heater motor. It all starts with the drain holes which block up ten starts to rust out the bulk head and water then makes its way behind the dash into the car.
Not an easy fix but things can be done to fix it, I have just done a slc which was really bad.
Take the cover off over your heater motor and have a look what it looks like, also lift the 2 grilles under the windscreen and check along the panel work and also take the under panels from the dash and look under with a light and you may see whats going on.
I will try and post some pictures tomorrow and some more info.
If you would like to call me to talk over this in more detail then PM me and we can swop contact details
also in addition to the plenum chambers, check that the box sectioon that runs from the rear of the passenger front wheel arch hasnt rotted though. check this by looking up around by the back of the glovebox from inside the car - should be able to see some spot welds - check for rusty holes around here. and remove the battery tray to check from the other side.....