I don't have the wiring diagrams, but the back speakers on my 1995 E320T also failed. The dealer found the problem within the speaker cable that runs near the base of B-pillar (just left to the driver's seat anchors).

Seems that during assembling, the cable got cut by an interior trim and eventually cracked. Due to this mishap, the cable shorted out the amp circuits for the rear speakers. Under warranty, the cable was changed and the radio head united was wrepaired.

Later, the dealerfound out that this problem is common with E320T (W124s).

A. Rosich
S320, 1998
E320T, 1995