I'm not sure if this also applies to your W123, but in the early W124's with the one-piece radio, the wires for the front speaker run forward and plug into the speaker harnesses just behind the radio. If you end up not wanting to run new wires, you can tap into the front wires here. Connect the rear outputs to the wires that run to the fader, and just run the rear channels through the fader control (leave it in the center position - this should be a metal-to-metal contact that will give you better SQ).

This way, you get full fader control through the head unit, but without having to pull out a bunch of interior trim and running new lines.