Dear Cees
Congrats for your Sherlock Holmes discovery. Grounds are always forgotten. Modern Mercedes and BMW have evolved significantly on the quality of the grounding post. Those are special surfaces highly polished where the terminals are tighten with 10mm screws and are always raised from the rest of the body structure.
I think that time has teached the designers that grounds are always overlooked and cause many erratic problems on the different components of an electrical system.
I would recommend to all you that pay attention to your electrical stuff, to polish each of your grounding post on your loved MB classic. On the pagoda there are not many, but as far as I remember:
1. Main ground post on the cabin behind the heater controls, accessible if you remove central instrumentation and steering wheel
2. Ground post on each side of your radiator, on the lower part. These ones are responsible of headlights.
3. Ground post on the right fender, as mentioned by CEES, responsible of the wiper motor
4. And the. MOST important the GROUND STRAP between transmission housing and the firewall. This ground allows the starter to work at its maximal speed and also brings ground to many electric selenoid or valves installed on the engine to run the cold start system, among other functions.
5. Also in some cases there is a ground post next to the left fender relays arrangement that allows those components to work properly.
Always sand the ground surfaces. Start with a coarse grit like P80, and finish polishing the area with sand paper grade P180.
APPLY CONDUCTIVE GREASE, no matter if it looks terrible or tarnishes your hands, it is imperative to use CARBON CONDUCTIVE GREASE for those ground terminals. Only with that you can assure that corrosion will not build up between cable terminal and the grounding surface.
Of course the BATTERY NEGATIVE main cable ground to Chasis should be in acúlate clean.
Here a link of the conductive grease I use in my projects:
MG Chemicals - 8481-1 Premium Carbon Conductive Grease, 3 fl. oz, Black
https://a.co/d/bsFErcoSold in Amazon.
MG Chemicals - 8481-1 Premium Carbon Conductive Grease, 3 fl. oz, Black
https://a.co/d/5CExfPjBest regards
L.Peterssen