Don't expect much from replacing windscreen seals. Both have been done on my '84 300D and, I've added sealer (the special $20/tube 3M stuff made for windshields) to the front windscreen with no detectible change in leakage). On closer inspection, I found a couple of suspicious places in the cowl/hood hinge area (see this
link) that I addressed but she still leaks on the drivcer's side.
I know that the pinch weld underneath the center of the rear windscreen is pretty badly rotted out yet starngely, she shows no outward signs of rust. I'm sure that this is the leak into the trunk (sitting still an in a heavy sustained rain, into the rear footwells).
I've been through all the body drains and roof drains with no success. This car sits in a garage both at home and when she gets to work. My solution for now is to park it when there's a chance of rain and have SWMBO drive the beater SDL instead.
One of these days, I guess I'll have a body shop rebuild that pinch weld.