I'm pretty sure I've done this after pulling the radio and ashtray, don't remember if I pulled the glovebox or not. Might have, as that "makes" a great big hole you can see and access
There is a bracket that holds the speaker in place between the two defroster vents that is held in place with nuts.
I lay on my back in the floor with my feet up over the back of the seats to see up there.
Slow and easy does it or you'll think you stuck your hand in a mulcher.
5/16 nuts if I remember right. (3/8 maybe)