W204 C63. The bleeding windscreen wipers squeak and I have utterly failed in my attempts to do anything about it.I have tried umpteen different makes of wiper, including Mercedes own, still squeak. I have cleaned, clayed, polished the screen. Still squeaks.
My next attempts are either replacing the screen or replacing the wiper arm linkage. Based on a theory of either the screen having been etched somehow (under street lights, you can see it's covered in microscratches) or the linkage having too much play in it.
Long shot, but have you checked the wiper arms? It's possible that if they've been twisted slightly, so the blade might not be held exactly perpendicular to the screen, which can cause them to squeak.