You probably have aftermarket speakers. If someone fit an aftermarket Kenwood deck in your car, which came with the infamous/famous Becker BE1432, it is likely that they spent a lot of effort to fit that Kenwood because the the BE1432 has lots of proprietary wiring / main unit in the trunk / 2 amps in the trunk that may have been bypassed with aftermarket wiring. (test - does the fader wheel work?)
If someone went through all that effort, they probably fit aftermarket speakers.
Another clue --- do you have the original MB phone? If so, turn it on. If you don't hear the "braaap" upon power-on, then someone has replaced the driver's side dual voice coil dash speaker with an aftermarket speaker.