Front Window Sun Stain

I recently bought a 2004 convertible and the front window has a stain that I have been unable to remove. It covers a lot of the window and can mainly be seen when the sunlight hits it. It's almost as if it was stained by the sun. Is this possible and can it be repaired? I have tried a lot of different cleaning products that have had no effect. Any advice before I have to replace the entire window?