On my 1993 300ce, the LEFT front speaker intermittently went in and out, and often made popping sounds. I swapped out the fader switch with a known good one - no change. I inspected the euro speaker plugs that attach to the amp in the trunk, and discovered that one of the wires had come free of however it was attached to the male pin that gets inserted into the amp. I spliced in a good euro plug, connected it to the amp, and clear sound emerged from all 4 speakers. Hope this helps someone.

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87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)