You don't need to change the valve, just the insides. Buy a rebuild kit. You'll see it between the inner and outer firewall, just a bit to the passenger side of the middle. Heater hoses lead to it on from both sides. The kit you buy has a piece of paper that describes how to change it, or at least the one I bought did. It takes more time to find all the right tools and drain some antifreeze than relacing the parts. A really simply DIY job.

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Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport