Spent some time testing the existing speaker plugs behind my head unit and found that two of the speaker wire sets lead directly to the rear door speakers and are not affected by the fader switch. This is not what I was expecting to find based on other threads around bypassing the fader. Perhaps the early W124 wagons are unique in their speaker wiring.

I am able to run the left and right rear speaker outs from my new head unit to these wires and the speakers behave as expected, i.e. I can use the head unit balance and fader.

For the fronts I will need to run new wire if I don't want to cut existing wires going into harness that leads the fader.