Whatever you do, DO NOT buy the POS subwoofer(as they call it) from the Fiero Store. I bought one and sent it back. didnt mount properly, seal well, and was just a regular old speaker. BUy Rodney's if you want to replace it. Heard nothing but good about it.
As for the screws. It was a b*tch for me also. Sears has those standard socket wrenches. Looks like a regulat screwdriver, with an open socket on the end. They have a thinner wall then most sockets and will fit right in those four speaker end holes with ease. Just remove the ones around the perimeter, then pull that speaker and cover off. Take it to sears, and use their wrench to pop it off. you can then buy yhe wrench,OR just go back up there when you want to replace the speaker.