Voltage regulator is fairly easy to get to and is a common culprit. one other cause I have seen is a sticking glow relay. When these stick, the amperage draw exceeds the alternator output. The glow relay is mounted on the driver's side of the engine compartment and is black with a removable top. Check the voltage regulator first. Good Luck!
Mercedes Star Technician,
Mercedes Master Guild Technician, ASE Master Tech. 18 years exp.