Dave, just have a look at this (google images "W126 Soundsystem", first 2 rows of pics): http://www.google.de/search?q=w126+soundsystem&hl=de&client=firefox-a&hs=j3o&rls=org.mozilla:de:official&prmd=imvns&source=lnms&tbm=isch&sa=X&ei=U98HUKekIImTswbj-6j4Ag&ved=0CD4Q_AUoAQ&biw=1280&bih=627

or look at this (text in German, unfortunately): http://w126-archiv.mercedesforen.de/index.php/Das_Soundsystem (Summary: the sound systems consists of 10 speakers; in addition to uprated speakers in the dashboard and the rear shelf, two woofers are installed in the front doors. The sound system is powered by two amps (one for each side) that are installed behind the rear seat. The entire system is made by Becker).

Most info is on the German W126 forum where I am a very active member 

You are right, however, in observing that the front doors were never delivered with stock speaker locations in the front doors (as is told, the then head of design Bruno Sacco said this would have ruined the clear style of the door cards). The sound system (being heavily priced) was ordered in low numbers. However, three of my friends do own this Sonderausstattung (I don't, but have upgraded my system with a Becker satnav radio, new speakers, new lines and a Blaupunkt 400W amp for the rear speakers).

Dave, a W126 is almost never fully loaded as there were so many Sonderausstattungen available; trip computer ("Reiserechner") is a very sought-after gadget, for example. A friend of mine even has the rare fridge in the boot, another one the folding rear tables in walnut. There is a mini fridge available for the rear seat (in place of the middle arm rest), an option for moving the passenger seat electrically from the back seat (in order to get more legroom), cardanic reading lights in the rear... this list goes on and on, a (not even complete) SA list can be found here: http://www.w126-wiki.de/index.php/Sonderausstattungen (this Wiki being a project where I am largely involved).

You can be happy you do not have the HPF hydro suspension - it rides very well, but parts are getting very scarce for this specific SA. Owning a 420 myself, I dare say this is one of the most economical W126 models, since the smaller straight 6 engines are revving too high and the bigger 500/560 have a more complicated rear axle that is very expensive when it comes to repairs.