As explained by the M-B's Tech Advisor, the wire routing for the rear speakers on all W124 (S124) wagons tend to give this type of problems.

Lets start by the location of the rear speakers, which is not the best (on the rear armrest, completely blocked when the rear seat is occupied).

W/S/A124s cars still come from an era when M-B did not care at all for Hi-Fi. I guess that is why sound quality, speaker wire routing and speaker location are not a prime factor in their design.

A. Rosich
S320, 1998
E320T, 1995