Mercedes-Benz: Leaky 300SD

During heavy rain the floor in my '85 300SD get soaked. I read that the
windshield seal could be a problem. The seal, or weatherstrip or
whatever it is has gaps in it, I can stick a screwdriver inside, between
the strip and the windshield. I was told that that is normal (??), is
that true? Doesn't it have to be completely sealed up? Also, at the two
bottom edges of the windshield there is some openings at both ends of
the rubber seal/strip where a finger can be inserted beneath the
windshield bottom, does that have to be sealed up?

Besides the sunroof, where else should I look for the source of this
leaking, all floor wells get soaked.

Thanks in advance for any help.
cp

Of course the windshield could be the cause, I wanted to be sure
something simple (and free) wasn't the cause.