the stereo in my 91 300e was a becker cassette deck and radio
the unit in the dash was a cassette deck and controller in the trunk was a main unit that is the tuner and preamp controller along with a left and right channel amp they are all on the passengers side in the trunk the fader in the center console controls dash and rear deck speakers
the dash speakers have a capacitor and a choke on them to act as crossovers for the dash speakers
i replaced the deck and dash speakers and added a small amp for the door speakers i wired the deck directly to the new dash speakers and to the factory rears you can get these at the fader i put the amp in the same location as the factory controller in the trunk and grabbed the wires to the factory doors there i run the doors as mid woofers off the amp and the dash and rear deck off the deck it is a nice upgrade without going crazy run power for the amp to the battery fuse it at the battery you have to run an antenna cable from the antenna to the deck as the factory goes to the controller in the trunk