Interesting. I have something that looks similar on my rear window. I suspect it might be from autoglym glass polish that I failed to fully remove when i cleaned by car the other weekend. Have you used a glass paste product?
I'm going to see if more of the same stuff can take it off--invisible glass already failed.
Interested to hear if you have any success. I'd try clay before steel wool, assuming you don't have tints.
2009 e90 sedan | sport pkg | navigation