It is either the back window of a convertible falling down or the headliner of the coupe falling down. Either way you go, get ready for a repair. It is a shame! When I had my headliner replaced in my coupe, the guy who did it (in business for 40 years) said he never saw a headliner fall down so fast on a new car. He said the glue must have been defective and they didn't wrap the material properly around the edges so the heat and moisture got in there.

I had a 2000 Convertible Camaro with a back glass window that NEVER fell down and I NEVER had a problem with it.