1) I believe that the 20 mm rubber expansion plug behind the L headlight covers what was actually a paint run-off hole, but you're right that water accumulates there. In my 1988 260E, I pulled the rubber plug out to let the puddle of water drain after washing one day and the plug tore apart from age. As it was the original plug, the underside was completely coated with the PVC rust protection sprayed on the undercarriage at the factory; that made it rigid and brittle. The replacement part # is 113-987-00-44. Now when I wash the car, I just pop out the plug, let it drain, wipe up around the area (particularly the 0.2 ohm resistor for the aux fan) and reinsert the plug.

2) I can't identify these pieces from the photos, and you don't say where they are located in the engine compartment.

3) As for the surface rust behind the battery, I'm not sure what metal area that could be, b/c there should be just the plastic electronics cover there to separate the battery from the relays. It appears that at some point that metal panel was scuffed up and developed surface rust from ambient moisture. In any case, that rust looks so superficial that after 21 years I don't suppose it's going to be a problem now. I'd be inclined to just clean it up as best as possible, put a heavy coat of carnauba on the area and keep it wiped dry after car washes. That's not going to undo the rust, but it'll help keep it under control. Of course, if you're really worried about it, a permanent solution is obviously possible. Take it to a body shop for rust treatment.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't