Seen many of these on Benzs mostly about 15 years old or more and usually on W124s. I steer clear of these when looking to buy these cars as the salesman will usually say that theres nothing wrong with the rear windows, just some window stains which will probably wash off easily, not to worry about them. They won't tell you that it cannot be washed off ever and how much it would cost to have it replaced and they won't do it just for the sale unless you pay for it. Just saw these when I was out looking for a W124 and went to car yards as well as at a repair shop which sells spareparts as well as some used cars too. Ended up buying a W202 instead and because this model is a lot younger( 8 years).