Check on the power amps
The 1991 560SEC (10 speakers from the factory) routs signal from the front end Becker unit pre-amplifier to power amplifiers located behind the rear set back (that's how it is in my 1990 560SEL, and I suspect your 1991 SEC s/b the same). The output from the power amps powers the speakers.
You may want to check that the power amps have not been by-passed. Is there a separate power amp installed somewhere? It may also be possible that your mid-range speakers and tweeters are blown from excessive current, though this is more likely if after-market power amplification was used.
I suggest you inspect the tweeters (in the dash under the windscreen) and the mid-range speakers (in the door panels) for damage first, then check if the OEM power amps are in circuit - you will have to remove the rear seat back to get at them.