Sounds like either the amp blew, or the speakers blew, or both, or none at all or its just a faulty wire somewhere.
Rule out the fader switch, and if you can, get a battery, I usually use my Makita Drill Battery and touch the positive and negative from each spaker to the positive and negative on the battery. What you will hear is a pop like sound. That is how stereo shops trace the wires to the speakers, and its a good way to see if your problem is the radio or speakers.
With the speakers ruled out as the problem. Bypass the fader switch and see if you get sound directly from the radio to the speakers. if so, then we know the switch is faulty. If you don't get sound, it might be that the radio's amp has failed.
If its the stock becker stereo, some of what I have said may not apply. It applies more to aftermarket head units.
You might also try posting about this in the car audio forum, there are lots of knowledgable people in there who know a lot more than I do about car stereo's.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.