[W126 Coupe] speakers

[Dick Spellman]

The factory rear harness goes from the receiver to the amplifier which is
typically under the passenger footwell firewall.  This does not matter.  You
should simply route the new wire from the new receiver so that it flows into
the plastic raceway to either footwell, under the seat channel and up the
slope on the back seat where it needs to pass through a small channel
avoiding the rim of the rear seat where it meets the slope.  Then it's just
up a short distance on the rear seatback and through the wiring grommet, one
to either side of the car.

Even if you plan on powering all speakers from trunk mounted amplifiers you
need to observe the same routing for the signal cables.

Good luck,

Dick