Negative, it's not the windshield gasket.

I went through this for years with 123 cars. I tried a number of things. Finally a few years ago when I found mercedesshop.com, I found the glorious answer. It came from a fellow in Sweden who offered the solution as if it were just common knowledge about 123's. Anyway, here goes.

Raise the hood and look at the area directly underneath the hood hinges. At the rear of this area, you will see a sort of dent in the sheet metal which leaves a drain hole between the sheet metal directly below the hood hinge and the firewall. Clean this drain out with a high technology tool called baling wire. For city dwellers, a coat hanger offers an equally high tech substitute tool.

Do this on both sides while you're at it, even though your problem is currently only the right side. Now, you will notice that the vertical piece of sheet metal that separates the hood hinge pocket from the engine compartment has an opening at the bottom allowing overflow from the hood hinge pocket to make it toward the engine compartment. Behind this point which is under the master cylinder on the left side and under the battery on the right side, you will find two more similar drains. Use your high technology, hand crafted tool to clean out these drains. If you are able to locate and clean ALL FOUR of these drains, I will offer my personal guarantee that this problem will disappear, at least until the drains clog up again.

It was worth the effort to find mercedesshop.com just for THIS tip. I bet over the years of driving 123 cars I baled a hundred gallons of water out of the back floorboards before I learned this fix.

Good luck,