I bet you a dollar I know exactly where its leaking from......... Its from that Onstar antenna or whatever on the front of the roof by the sunroof. Its an easy fix, all you do is unbolt it and apply sealer on both sides of the rubber gasket and bolt it back down and wipe off the excess sealer. I used an RTV sealer for a water pump on my GFs Brothers 04 CompG. Its been that way for 9 months without any issues at all, and the car sits outside too.

Good luck...