Yes. Replace the rear window as I have to do in the Blue Bird. There is no other fix that I am aware of.
Should be able to find a rear glass in a Junk yard. Albeit, coupe rear windows are few and far between, but attainable.

Yes have a local auto glass shop pull yours and replace the new one. +2 on the new seal. And be ready to repair any rust(cause of the cloud) under the seal. POR 15 is the Best IMO.
Depends on if you have the defrosting rear glass what the cost will be. A 500/560 SEC rear Glass from a dealership is $1300.00, one from a JY is maybe $50, but you have to pull it. Easily done because you don't have to worry ablout saving the seal. just un hook dfrom the defrost connectors inside, pull the trim and cut the seal to expose the glass. I'd cut from the inside also jsut to make it easier.

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.