I am surprised there are no rear speakers but....
If your W126 has the two amps mounted behind the seat you really do have a lot of flexibility.
The amp(s) the salesman is talking about are inside the cabin behind the rear seat back, which is easy to get off to check. Two latches on the seat bottom and 3 screws along the bottom of the seat back. There should be two 2-channel amps. One feeds the left F/R and the other feeds the right F/R.
If you can find room for one or two new small amplifiers behind the seat that is best bet because all F/R shielded lines from the fader run in there, and all four output lines are there as well. Then all you may need is an extra fused power line for the amp (current amps combined run off an 8 amp fuse so you decide :^) If not, you could extend the lines into the trunk and mount an amp there. Just note that all the wires running from the head unit to the fader and then to the amps, are shielded so use shielded wire.
Do not get too large an amp or you will end up having to run new speaker wires (35 - 50 watts/channel RMS should be fine). When looking for replacement amps get ones that have both line-level inputs and speaker level inputs in case your MB head-unit signal is too strong.
Some options to minimize wiring and cost:
1 - Easiest: Buy a quality set of coaxial speakers that fit into your front doors. Disconnect dash speakers. Put in a 100-200W 2 channel amp (use speaker specifications), and hook up the Left-Front and Right Front wires. Set fader to front.
2 - A little more work but much better sound: Buy a nice component speaker set (e.g. Rockford Fosgate or similar) that comes with a passive crossover. Mount the tweeters in the dash and the midbass in the front door. It is a little more work because the component set comes with a crossover which separates high and lows. The crossover can be mounted anywhere but you'll need find the separate dash speaker wire and door speaker wire to hook to the outputs of crossover (or run new wires). This may require a bit larger amp than option 1. Set fader to front.
I do not bother with rear speakers - spend the money on high quality front components that make for a nice "sound stage". This also frees up your rear signal lines for sub(s) in the future. 200-400 watts of sub power really improve sound for most music. Just add another 2-channel amp using the L/R rear lines form the fader as input. You can use 6" deck mounted free-air subs, a powered bass "tube", or a small sub box with 8" sub(s). Set fader where you like.
These options compromise some because of the unusal MB fader/signal setup and, using existing wiring but it will sound good. Next step up is to ditch the stock head-unit, run pre-amp coax cables to a fat amps/crossovers in the trunk and run larger gauge speaker wires.
Running wires is fairly easy in the W126 because it has plastic wire guides under the mats. Also try to use RMS power ratings rather than "peak" ratings for the all components you buy.
Search Crutchfield.com site for thier "Car Stereo Installation" guide it is very useful.
face=Arial size=2>...<BR>> > I have searched for other messages
addressing this with no luck. I just<BR>> > bought a 1989
560SEL. Doesnt take much volume to cause the speakers to<BR>> >
rattle. maybe its overloading or maybe the speakers are shot. Went
to<BR>> > price an upgrade and 3 stores so far have treated me very
poorly. Guess<BR>> > they saw MB and just wanted me to hand over
my Visa to the tune of around<BR>> > $1000.00 One wanted 1100.00
for a 350 watt system and the other two<BR>> wanted<BR>> > a like
amount for around 1000 watts.What is your experience? I'm
thinking<BR>> > that I would like to have a system that puts out about
double what the<BR>> > factory Becker system does. The audio shops
say that my amp is "behind<BR>> the<BR>> > seat". Does this
mean I can replace it with a more powerful amp and<BR>> upgrade<BR>>
> my speakers and be happy? I suspect I'm dreaming. What can
you tell me?<BR>> > TIA<BR>> > Jack<BR>> ><BR>>
><BR>> <BR>> <BR>> </FONT></DIV></BLOCKQUOTE></BODY></HTML>