I got similar setup on my 95 E300D... It has an aftermarket head unit
installed by previous owner... It has a factory amplifier in the trunk. From
what I can tell, the amp is a 4 channels amp.
But whoever installed the system, they really did a hack job... the front
door speaker, they replaced with one size too small... 5.25 rather than the
6.5"... yet that pair of speaker does not work...
So there must be a crossover somewhere that I am not sure where...
To get 4 speakers to work on two channels, you need to have a crossover.
As for your rear... they basically installed a bass blocker... which can be
obtained from partsexpress.com... they prevent the speaker from playing too
low of a bass... that will distort the sound and ruin the speaker. for
6.5", it should have like 80 or 100 Mhz bass blocker... block any
"subwoofer" frequency range.
As for the front, I would want to trace the wires to find the crossovers...
or check for mized up wiring by the head unit if already aftermarket....
where installer could have messed up.