Check Codes, where are all of the chassis ground on the W208? I've asked this question before on forums trying to (possibly) track down my 'dimming lights' issue, thinking perhaps there is more than the single main ground in the trunk/boot off the battery. Checking/cleaning others if they're large/major points of contact is my next step, but I've never heard anyone who can tall me what points these are. Thanks!
There are two ground points under the hood, one on each side for each set of lights. Usually on the front side of the fenderwell, it's just a small stud (4mm?) with several brown wires coming to lugs that are stacked on the stud and secured with a nut. With the key in your pocket, remove the nut, use a mild metal-bristled brush on the stud, lugs and underside of the nut, and then reaffix them all, tightening firmly.
There are various other ground connections on the car as well. The engine ground strap is often overlooked, but it can certainly loosen and literally wear out as well. The connection from the battery to the chassis (the one you mention) is also of obvious importance, though problems with these latter ones manifest themselves in ways other than just dimming lights.
If all of your lights are dimming, I'd spend a little time looking at the alternator and regulator. The brushes could be getting worn and hanging up a bit. If the alternator sounds okay through a steth, then were it my car I would be inclined to pull the alternator and replace the regulator (it comes with new brushes) inspecting and cleaning the commutators while I was doing so.
Hope that helps, good luck to you.