The radiator drain valve is at the bottom of the radiator, literally right where your leak is. You should be able to see it if you remove the Belly cover under the engine and look up at the bottom of the radiator. At least that's where it is on my car. a '95 E300D. Should be the same on yours.

Sure hope that's your problem.

Good luck

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'95 E300 Diesel, 264,000 Miles. [Sold it]