Interesting question. I'm not sure it's a plastic. I've heard it referred to as glass resin, and I think it's similar to two-part epoxies. Those are polymers, right? So I guess chemically they are at least close to plastics.
Glass resin is fiberglass. You have a resin and a hardener that are combined and then the woven glass mat is saturated with it. Resin makes it stiff, fiberglass is more or less 'rebar' for tensile strength. Yeah, it's like epoxy. Exothermic reaction and all that.
But I know nothing of plastic molding. Don't they just heat the plastic, inject it, and let it cool? So it's gotta be a totally different product. I've seen stuff kinda claim to be both ('Duraflex'). Supposedly fiberglass but with flexible plastic. Not quite sure how that works.
I've also seen plastic parts with glass fibers just thrown in them. Have a friend with an AR-15 lower like that...gotta think it's little to no different from just having one made of ABS plastic.
Anyhow, my main point was just that some parts are plastic (bumpers), others are fiberglass. Were the bumpers fiberglass, you wouldn't have that crazy flex in the rear fascia (see Top Gear review of C6). And some, I got no clue. I know the floors are/were steel, was thinking it would make sense that the quarters are, too. But historically, Corvettes have fiberglass bodies. So no mending them with toothbrushes.

Fiberglass resin is actually a lot easier than welding plastic.