Here's a link to a post I did awhile back, documenting the install on my 93 W124.

The long-awaited W124 budget stereo install (lotsa pics)

Did an aftermarket head unit, 6' antenna cable to the rear, and spliced into the speaker output wires from the OEM amps.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car