This is what I am thinkin'...if the alternator was dead, or very close to dead...obviously, you will have a dead battery after a while, which makes sense...but in your case...it seems the alternator is not quite providing enough amps/not performing up to specs and is almost on its way out...so although it is still charging the battery and powering your car electronics, it seems to be really struggling when you turn up the volume on your stereo, or keep the headlights running, etc. The battery is still being charged though...but maybe not being fully charged...but enough for starting the car, etc.

One way to test is to drive around your neighborhood for 5 minutes or so, until you see your headlights dim down as you described...and have the radio volume up or whatever electrical drain that you have in your car...then, pull over to the curb...put the car in park...and rev the engine up high...see if the lights come back up in brightness...

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1995 black pearl/black Mercedes-Benz E420 :