A friend with an '04 had water in their maplights and it was caused by a bad seal on the fin-like Onstar antenna on the roof. Dealer warranty replaced the entire assembly and all has been well last I heard. Not sure if that antenna was carried over to the 08's though, but all I could think of when you mentioned that. From what he told me, the dealer knew immediately over the phone what it was and said it was a common issue they had come across.