The raw end of that cable wicks moisture into the copper stands. This will happen to any wire with raw ends exposed. This causes corrosion troubles, particularly in the roadsalt belt. I believe this one goes 'uphill', so gravity also works against it.
The ground wire on the 124 is on the thin side already. Loose battery mounting or twisting of cable can further stress it.
This is a common problem on many cars, at least in the NE. Fully exposed braided 'ground straps' have got to be the worst design of all.
If you excavate into it, you will probably find only the first few inches are corroded.
How to prevent?
When replacing cables, I like to 'wick' solder into the raw cable end(s), then seal tight with heat shrink tubing or liquid electrical tape around the lug (or whatever connector).
'Tinned' copper cables are better for corrosion resistance, typically these are rated for 'marine' use. Fewer fatter strands are more resistant than many thin strands. Of course, if you keep the strands dry, it does not matter...
Clean and lube the ground contact area and fastener for good lasting results
1986 300E 5-Speed 240k mi.