I've had several W123's and they all had about half inch water in the floor pan after heavy rain, usually on the passenger side.
The drainage system off the roof seems to flow into a pocket around the bonnet hinge, where the lowest point has no drainage hole except the bonnet latch cable, which channels the water very effectively into the interior. What were the designers thinking?
I was thinking of filling that useless pocket with polyurethane foam and making a drainage tube coming from the window gutter.
What about all the other millions of 123's out there, what do they do?
-VARIS

I went through the same chase for years on my 123 cars until right after beginning to post on this site, found the odd but real answer to this problem.

Open the hood and you will find four drain holes that must be cleaned out with baling wire (a suitable substitute if you are a city person is a coat hanger. ) Start on the drivers side and look in the hood hinge pocket. You will see a sort of dent in the fenderwell sheet metal as it meets the firewall sheet metal. Use your high tech, baling wire cleaning tool and clean that out. Do the same thing on the passenger side.

Then look for another such drain inboard on the other side of the vertical metal that separates the hinge pocket from the engine compartment. On the drivers side this will be under the master cylinder and on the passenger side under the battery box.

Also see that the opening at the bottom of the vertical metal that separates the hinge pockets from the engine compartment is clear.

This sounds crazy, but that is where the floor board water typically comes from if these drains are stopped up. I went years bailing water out of my floorboards before learning this from someone, if I remember correctly, from Norway. I've been a believer in mercedesshop ever since.

Good luck,