You can get the ESP/BAS light on for loads of reasons since it covers Brake Assist and ESP systems each of which have numerous sensors. You really need to hook it up the an MB Star system or similar at a dealer (you may be better off at an MB dealer for this) to get the codes from the CAB module. Assuming that calibration isn't the problem as posted earlier, then the problem is most likely a sensor (brake travel, break pressure, wheel speed, Yaw etc) or an intermittent connection to one of the sensors. Check the service manual for the full skinny on all the different sensor inputs that BAS and ESP uses. The CAB will also throw codes for under and over voltage which means the problem could be elsewhere, such as a corroded supply fuse contact or relay contact. Unfortunately you really are in the dark without knowing the codes that have been stored. If you can get the codes then maybe I can be of more help.