I have a switch like that on my SL. With the switch on (the red light lit), you get a firmer ride. I leave it on always because I like how it handles that way. I haven't figured out how it makes a firmer ride yet, but I think the back shocks fill up a bit more or the front drops a bit.

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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