Guy,

However many speakers you have, they are ganged to an equivalent 4-ohms per channel; front left and right, rear left and right. Each front channel includes a dash speaker and door speaker set.

Run speaker wires from the new head unit to the back seat area and tap into the wiring harness at the amps. I can help you identify wires if necessary. You'll lose the function of the fader switch behind the shifter but it's a lot more work to keep that functional.

As I understand it, 6 speaker systems have a single driver in each door and rear deck location, 10 speaker systems have a separate tweeter and woofer arrangement in each of those locations. Plus the pair in the dash.

Models without speakers in the doors have a pair in the dash and a pair in the rear deck. I think the W126 series in the US had 4 speakers through 1987 and got door speakers and external amps in 1988.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD