On my 1986 760, there were actually TWO seperate amps, thankfully, they were connected to one another. They can be found if you take off the black plastic panel where the driver's knees are. The system that is used to route the signal is pretty complicated, I ended up pretty much rewiring the whole system. I had to cut apart the thick black wires that hav many wires inside of them, and separate these wires to find out which ones went to which speakers, and stuff like that. Its a pretty complicated job if you do it yourself, but if you've installed some stereos before and have some understanding of how that stuff works, it will take you a little while, but you can do it. I don't know if I have the same stereo as you, but I also had the external Equalizer, which complicated matters even more.
From the head unit, the signal goes to the first amp, out of the first amp, through the second amp, back to space behing the head unit again, where it finally splits into the four different speaker, maybe different on yours since you have six speakers, but probably some similarities.