Got the new bulb (original Osram 2 pack P21W 7506 from Amazon) and installed yesterday. Went through the following steps:
1. Removed an existing bulb from the right (passenger) side in the top row and inserted in the place of the burnt one on the left (driver) side bottom row. The error now says: "check right fog lamp", the "right side brake/tail lamp" error is gone.
2. Put the*bulb back in its original position, fog lamp error gone, back to "check right side brake/tail lamp" error.
3. Inserted new bulb in left side bottom row. Now all errors gone.
Conclusion:
1. Top row bulb is fog lamp, bottom row is brake/tail light.
2. Doesn't matter which side the bulb is burnt, MB keeps reporting as right side!
Also, I handled all bulbs through a cloth to avoid directly touching the glass. Don't know if this is needed since the bulbs look like incandescent, but took the precaution anyway.