You may also have not bypassed the factory fader. If I remember correctly, the factory setup uses a common ground for the left and right channels, while aftermarket units do not, and usually incorporate the fader in the head unit.

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Hanno
'79 6.9 Sold (after 27 years)
'83 280SL, 5 spd.
'94 E320 Sdn. 5 spd conversion
'02 E320 Sdn.(on loan to mom!)
'87 300E (5 spd. conversion) Sold
'05 E500 Wagon