It sounds like the battery isn't being charged by the alternator, and as if there is also a short or current drain somewhere. My 87 300E drained overnight before. The alternator needed new brushes (regulator is built in the brush assembly), and the current drain turned out to be the rear seat dome light. The bulb was burned out, and I didn't see that the dash switch was in the on position. This mean t that the other 2 bulbs on that circuit in the lower rear doors were on all the time, but I never saw them. So they drained the battery overnight.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E