There should be one amp for each side behind the rear seatback. The fader controls a preamp signal.

The stock stereo sends a common ground signal to the amps and sends floating ground power to the speakers. That's only two wires into the fader switch and four wires out of the fader switch. I wouldn't risk wiring your stereo's speaker outputs for common ground but I think you discovered that you can use the car's fader switch.

Looks like you have to route speaker wires from your stereo to behind the rear seatback and hope that the speakers are of an impedance your stereo can handle.

If you need help with identifying wires at the amps, let me know.

If there are any crossovers, they are part of the speaker assemblies.

Sixto
91 300SE
87 300SDL
83 300SD