It's under the windshield washer tank. I changed mine out last week. Disconnect the top 2 hoses on windshield washer bottle and remove it. It slides right out. When you look straight down you'll be looking at your auxiliary water pump. There are 3 hoses attached to it. Remove the hoses, disconnect the pump from it's bracket which is also wrapped in rubber; replace.

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1995 E420 152k
1991 300D 2.5 348116 daily driver
1992 300E 2.6 196k-work in progress(head gasket)
2002 BMW 745Li 93k