I replaced this part myself a few months ago when my locks stopped working. I believe it was less than $100 used. It is for the door locks. Make sure there are no other leaks in the doors when you replace it or you will very quickly burn out the new pump.

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1996 (W210) E300D 86K - Traded in for a Lexus
1992 (W124) 300D 2.5 Turbo 202K - Sold
1983 (W123) 300D, 146K - Sold
1970 280S, 263K - Sold - Beginning of addiction