Run all four sets of speaker wires back to the stock amps located behind the rear seatback. Though you're going to be bypassing those amps, that location provides an easy access point to wires for all of the car's speakers.

Hope this helps...

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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)