Both of my slcs have this problem in the rear windows. I think it comes from the fact that there are defoster strips in the windows. The wires from these strips come out around the bottom and top edges where they connect to a copper strip. The bottom strip is connected to the wire that goes through the body to the electrical system. I think that where these wires come out of the window allows air to get in which may allow the sun to discolor the laminant between the two sheets of glass. Aside from replacing the window for a cool $1,600 I think that there is little that can be done.