Maybe some are both metal and fiberglass. From all the disscusion on going spoiler less and fixing these trunks, your is the first I have heard of being metal for '99-'04.
Lynn

 

You're all wrong (ish), It's not F/G but a Fiber fill mold with a metal bracing. I'm not sure of the materials exactly but if you've ever see a mustang burned to the ground you'll see the fibers and the metal.

 

i never even thought what material it is... but def. not metal or mostly metal... just went and stuck a magnet on and didnt stick... p.s. thanks for the new chip on my bumper... haha jk not really im not that dumb

 

I know mine is not any type of metal. I drilled holes for the spolier move back thing, and all ended up with a bunch of white dust everywhere. Looks like some kind of fiberglass to me also.

 

The deck lid is fiberglass, ive built my car from a mangled pile and had to fix the cracks in the fiberglass deck lid.

 

Great job you did haha. That is a really nice looking GT.