i wouldnt worry too much about the rears. u really want the fronts to be the main focus. u can get the mounts off ebay for $20. u hear ur music from the front not the rear.
http://cgi.ebay.ca/4x6-To-6-Nissan-240SX-s13-89-94-Speaker-Adapter-6-5_W0QQitemZ310010506153QQihZ021QQcategoryZ38646QQssPageNameZWDVWQQrdZ1QQcmdZViewItem