Factory wiring has two channel output to the shifter console. The fader splits the signal into front/rear with a common ground for each side.

I believe the crossover, if there is one, is built into the door speakers so you just have to run a pair of wires to each door. HOWEVER, each front channel is impedance matched between the door speakers and dash speakers wired in parallel so I would get the dash speakers out of a car with door speakers to keep the overall impedance at the appropriate level.

If you're doing all that work, get a recommendation for better front speakers than MB puts in the dash and door panels.

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