Front Window 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?

Windshields consist of at least three layers. Does the discoloration appear to be between the layers of glass?

Yes It appears that it is within the layers. It is really bad especially when the sun hits it, and I don't know what to do. Have any suggestions. Thanks