Did you adjust the volume set screw in the bottom of the amplifier? My 87 SDL had some issues that were resolved there. Take the amp loose from the fender and turn it upside down. You will see the adjustment there.

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Rick
Avid Benz watcher

Previously owned 126's: 2 1981 300SD's, 1987 300SDL, 1991 350SDL,
Previously owned 123's: 1978 300D, 1983 240D, 1985 300D
Previously owned 124: 1994 320TE

Currently:
2004 Toyota Sequoia SR5 206k
2010 Ford Edge 122k
Always looking for the next MB diesel!