Originally Posted by TX76513

On your W210 the voltage regulator is the brush pack. If you meter the + - terminals with the engine running your voltage should be north of 13.6VDC.
The brush pack is the likely suspect and its two screws to remove and not an expensive part. When you seat the brushes back in compress the brushes with a piece of dental floss, insert then pull the floss out and put screws back in.