Its the fader switch, controls what set of speakers will produce sound. Push it foward it will only run the front speakers, push it back and it runs the rear. Leave it in the middle it runs front and rear.

If you have an aftermarket deck most likley its been bipassed if you have the original deck it should work.

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Current:
05 E320 CDI
07 GL320 CDI
08 Sprinter
05 Dodge Cummins
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Previous
2004 E55 AMG
2002 C32 AMG (#2)
1995 E300
1978 300D
1987 300D
2002 C32 AMG(blown motor :[
1981 300SD
1983 300SD
1987 300SDL
2002 Jetta TDI
1996 S420
1995 S500
1993 190E 2.6
1992 190E 2.3
1985 190E 2.3 5-Speed