have a look in the boot, under where the rear speakers are, there are probably some screws holding the speaker covers on, take them out, remove the covers and then unscrew the speakers (probably from the top)
to remove the stereo take the plastic panel that surrounds the stereo off. make sure you've got all the screws out before you go tugging on it, often there are screws behind the ash tray so take that out too.
don't use screwdrivers to try and lever stuff off, you will damage the plastic. there will more than likely be stuff that needs to be unplugged, switches etc.
once you have taken the panel off, it will be obvious how to take the head unit out. if you need wiring diagrams for the stereo harness, check here
http://installdr.com/Harnesses/Nissan-Wiring.pdf.
please don't use the "twist and tape" technique, that's lazy and crap, and you could start a fire if your tape falls off.