great info! I have the same problem and a shop first told me to replace the alternator for $660 (parts + labor).

where shall one find a reliable site to order a VR?

 

So I have 81K miles on my car and I haven't had the problem since I changed the Voltage Regulator in the beginning of the year. I def. recommend doing this since it saves you a lot of money. I would call Mercedes and ask them what the Part Number is just so you get the exact one. I could not find one at a local parts store at that time. Maybe you can now. I just Google it online and found an actual MB part. Good Luck!