I'm only responding to second the advice Mattman has given.

I recently did the same thing in my 126 as he did: installed a new head unit with an internal 4x25W amp, ran 4 sets of speaker wires directly to the speakers bypassing both the fader and the two stock amps located behind the backseat. Results: Spectacular! Huge improvement in audio.

The only thing that I did differently than Mattman was to route all four sets of speaker wire back to the stock amp location (I got there via the center console, under the back carpet, and then under the backseat. Since wiring for all drivers meets at that particular point, it was a breeze to tap into the speakers from there.

I've been wrestling with whether to upgrade my speaker system; but honestly, the vast improvement I've experienced just from the aforementioned rewiring has brought so much life to the stock speakers that I'm hardpressed to go to the hassle.

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Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)