Do you have an after market alarm system?

They are in my opinion the worst of the worst for draining batteries.

Minor update.
 

No, to the aftermarket alarm.

What I am hearing, loud and clear from several sources, is alternator or OVP relay. I will try to isolate them and report back. Thanks for all comments.

Pulled the OVP relay
 

Test light no longer lit.
So, I guess it is the relay, or something that is powered by the relay but not fused.
I will replace the relay and report back.

I believe the OVP fuse is a 10 amp... red one.

I saw one in a parts car once that had a 25 amp fuse in it. I wonder if this had something to do with the car winding up at a junk yard?

One of the things that Scares me
 

Other Previous Owners (Idioten) "Improving" the Electrical System...
On Anything! That Drives or Floats.

Thank God you don't see so much of these Self-Anointed Rocket Scientists in Aviation !

First thing to suspect is the OVP. My 124 gave me fits with dead battery issues. Rain water got into the relay from the cracked rain gutter above the battery & cover for the OVP. R2'd the OVP & made a raincoat for it with a plastic sandwich bag & a twist tie. That ended the problem. The other issue is a fuse in an unpopulated holder. Check your fuse chart & if is suppoda be empty don't put on in there. Many years ago with mine, I installed a fuse in a holder that was not normally populated with the intent to carry another spare fuse. Big mistake. Be patient, you'll find it! Let us know how ya made out.

This is what happened to mine. It was like a swamp in it.

Overnight Battery Drain
 

No one mentioned the most likely culprit which I had on a 124. On the passenger side in the trunk is the radio amplifier. It shorts and stays on thus draining the battery. Hold your hand on it after it has not been in use for a while, if it is warm, there is your culprit.

OVP tomorrow
 

Thanks for the input.
If the rain breaks, I will install a new OVP relay on Friday.
Will let you know.

Is this a standard equipment item? Wouldn't this be fused through the fusebox?

It was the OVP relay.
 

As many here suggested, it was the OVP relay.
I just replaced it, and the test light in series with the disconected battery cable no longer glows.

Interesting note; besides input from this forum, I spoke to a knowledgeable tech at an MB dealer w/ a 30 bay shop. He said ' If you bring me a 124 with a problem, i am putting an OVP in it. If it is more than 6 yrs old,
it should be replaced.

very easy. open the hood to the service position. Remove the battery (negative terminal first, then one bolt in front of the battery at the base. Battery slides forwarde one half inch, and then will lift out.) Pull upward on the weatherstripping that is closest to the winshield, in the area of the battery, which holds the plastic cover behind the battery in place. The relay is a silver rectangle, approx 1 x 1 x 4"", inboard, with a fuse in the top of it. There are several different mounts. Mine came off just by loosening an 8mm bolt, not all the way. connecter just came off without any fancy hold-on clamps.

Also, the ASR light is now gone
 

after replacing the OVP relay.

Thanks to all who responded.