The sub is in the center of the rear deck behind the seats. The cover pulls straight up with a bit of effort. It's an 8 inch sub, but it sounds pretty good for its size.
There's a hole under the sub that can be accessed through the trunk if you take the shroud off, but it's not useful in any way. You can remove the sub and work on it from the top.
There are foam repair kits if it's cracked, but mine was in good shape.