The mono valve is mounted on the firewall.
Look up ACC Mono Valve in Fast Lane for your car so you will see what it looks like. It is moumted with the shiny parts up with a electrical plug on top.
It has 2 water lines. One in the bottom and the other in the side. It is mounted high enough so that you will not lose collant.

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2009 CLS550 55,000
2004 ML350 144,000
2004 X5 95,000
2002 X5 165,000
1996 320S 155,000 (sold)
1991 190E 192,000 (sold)
1989 300SE 160,00 (sold)
1984 300D 210,000 owned since 85, (sold)
1984 300SD 160,000 (sold)