I replaced the rear deck speakers in my 2006 Mustang GT and from memory:
1. Unsnap the grills, just pull on them
2. I used a #7 (I think) socket driver Torx bit to undo the factory speakers
3. Unplug speakers in the trunk
4. remove speakers from the top
If you have a 2007 through 2008
stang, the whole top piece has to come off and requires disassembly of a lot of other stuff.
ETA: I actually have the Crutchfield supplied instructions that I can make a PDF out of, assuming that I don't violate their copyrights or anything.