I'm getting the impression the "cleaning" that's being referred to here is just dirty glass. Not the tarnished mirror damage that happens with age. It is way too hot to go digging through my storage sheds to find examples, but here is something similar I found online:
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On the rear view mirror it usually starts around the edges, particularly along the bottom edge and corners. And it spreads from there, growing with age. I suspect it might be from moisture getting behind the mirror and damaging the black reflective surface on the back side. Because it is not on the outside glass surface. This cannot be cleaned off or repaired. The only solution for it is to replace the mirror. The older the car the more common it seems to be. So later Bertones are not likely to have it (yet). Just wanted to distinguish what is being talked about.