Gary, I've got the same problem on my 190e.... I've come to the conclusion
that the body is rusting around the window.  Prove came this spring when my
front windshield cracked for no 'apparent' reason... of course it was the
expanding rust.  I'm going to try forcing some silicon rubber into the rubber
seal.

cheers, guenter

Not familiar with that specific model, but on my 116 300SD water in
that area was coming from the rubber seal around the tail light
assembly and from the fuel door.  In the case of the fuel door, they
used a horizontal vacuum driven pin to lock it.  There was a small
grommet around the pin that had worked out.  Just putting that back
fixed that one.  Really bad design though, had they located the pin
sloping the right way, water could not run uphill.

For the tail light, the best solution would have been a new seal.  But
the stealership wanted $75 for a new one, so I went with some black
weatherstrip adhesive, which solved it.  BTW, there was no evidence of
water inside the lens.  It was getting in behind the assembly, where it
fastens to the body.

To find the leaks, two suggestions:

1 - Get inside the trunk while someone sprays the car with a hose

2 - Place paper towels in strategic locations inside the trunk prior to
a rain so you can narrow down the area it's coming from.   You want the
towels to not touch each other, otherwise the water will just wick
along and they will all be wet.

Everyone is right... rear windshield leak, taillight seal leak... and there
is another source of leak... rust in wheel well... feel it with your hand or
see it with strong flashlight... rear wheelwell toward your trunk well...
usually rust there.

Yeah there are sunroof drains that get clogged and can be reamed out with
a straightened coathanger. Also, look around the taillights, even removing
them for a better inspection. That's a major source of leakage.