If you have a 1994-2004 Mustang, you'll need to remove the top backseat trim piece, the back seat itself, the package tray trim panel, and perhaps the rear quarter trim panels as well (not 100% on that last one). If you have a basic sound system, at that point you can just unbolt the rear 2 speakers. If you have a Mach 460/1000, you'll need to support the package tray from the trunk (it's held up from falling down into the trunk by the bolts), unbolt the package tray, and then you can get at the 4 speakers in the package tray.
Of course, if you have the Mach 1000, you'll need to remove the trunk amplifier rack 1st.