On the M117 V8 it sits under the other 3 bolt housing on the left of the water pump as you look at the engine. Follow the hose from the bottom of the radiator and there it is.

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1993 190E 2.0L (Euro.) sold
1991. 500 SEL (Euro.)
1991. 300 CE (Euro.) sold
1993. 500 E (U.S. spec.)