you do you have somesort of external amp powering up the speakers, right?
I have a 95 c280 it has a 9 speaker BOSE system (3 sets of speakers and a 6" min subwoofer) but before 96 they didn't label it as BOSE per say on the back of the rear speaker grills... the base speaker in a c class was 4 speakers powered by the HU.... the HU for both BOSE and non BOSE systems in 94-97 M/B (non fiber optic) are the same (either Alpine CM 2294/96 or Becker 1492/1692 depending on year and model... in your case most likely Becker 1492).
I cannot be absolutely certain about the setup on your E class without pics, but even if you have an 8 speaker Non BOSE then you must have an amp somewhere (most likely in the left side of the trunk behind the carpet covering) or at least active crossovers... I can't imagine the stock becker powering everything just by itself...

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95 M3 (US)
94 318is (EURO)
87 190E 2.6