My understanding is that there are two sound systems available for the W126. My 91 had the "enhanced" one. There is the head unit in the dash, and amplifiers in the dash and behind the back seat. There are speakers in the dash, front doors and the rear shelf. There is also another box located in the trunk on the right side panel. I did not want to interefere with any of the factory wiring, so I ran new speaker wire throughout the car. I replaced the head unit with an aftermarket radio. Many places now sell interface cables, so you do not have to cut any factory wires. This allowed me to completely replace the sound system without breaking anything else. I can also have the factory system in place and working very quickly come time to sell the car. The alarm system also ties into the stereo, so that is also a concern.
P.S. The unit in the trunk controls the power mast, so I left that intact, but pulled the signal cable so it could go to the new radio.
Crutchfield.com has been a great resource for me.