earth wire

Your earth wire is by the fuel sender unit. Access for that is through a small plate screwed on the floor of the luggage compartment - Yes you do have to take off the plastic kick plate and fold back the carpeting. Not a nice job, should you have rusty screws on the kick plate.

The earth wire is from the fuel sender to the floor body by the cover plate.

In that case as you realise it is the earthing for the head lights that are the problem. Unfortunately the wiring diagram does not show locations of earthing points. I think this information may be got from WIS (workshop information system). 

You would be better off cleaning all the earth common points. Starting at the front, the earth point are where you see a collective of brown wiring, usually there are quite a few bunched together and bolted onto the body. These can be found by the front of the inner wing closer to the back of the headlights. Clean the main negative lead from the battery terminal and the point on the body. After cleaning your connections, I suggest applying some Vaseline. This is a good electrolyte type of grease or gel. Good to protect battery terminals.

I will endeavor to get the proper earthing points from WIS, M/Benz owe me an hours session which I have not used as yet; if it is still on their system.