I have also had this exact problem with my W124 and I 100% agree with "amg280" that it is the OVP located to the right of your battery.

A suggestion, until you order the new OVP, you can remove the the 10A fuse from the OVP itself when you're not driving the car. This will save your battery from draining like I experienced. I was amazed at how that check engine light drained my battery overnight after a couple of nights.

Since the change, there has been no problems.