I just noticed brown spots on the bottom edge of the rear window and rubber moulding on my 2004 TL. I dropped by the dealer and they cleaned it off. No idea what caused it - anyone else have this issue??? It seems like 'rust' spots but there is no rust.

Demellie

 

Lots of us have that - it appears to be pollen or some other organic material. I too thought it was rust. I cleaned it by opening the trunk, spraying some silicone or detailing fluid in there to loosen the glop, and then used a soft toothbrush to work the material out, onto the glass, and then a paper towel to wick it up. I let it dry a few dasys, then used some compressed air (from photo shop) to remove the rest.

I also found that using a credit card to push down ont he weatherstripping for that area helped access as well.