E300 radio and volume

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Help! I have a '96 E300 with a troublesome radio. Sometimes turning the volume up decreases the volume of the radio. And sometimes turning the volume down increases the volume. My mechanic said that the unit could be refurbished, but eventually the same thing will happen. Is this correct? Is there a fix for this? Or is the fix a different type of radio/CD palyer? I realize that an amplifer lives in the "belly of the beast" and this is really a preamp/tuner. I do not have an issue with losing the trunk mounted cd changer. If the radio were changed out, is there a preferred manufacturer/model?

Although a fix of the stock unit would be easier, if possible.

Thanks. I really like the car, but question Mercedes' choice of electronic components.