You have to remove the back seat, the side bolsters, some of the trim and then take the entire rear deck cover out to access the speakers. The grills that cover them DO NOT come off.
Do a search. There have been some DIYs on how to get to them. Chances are, once you get them out, you'll need an adapter to put new ones in because the factory 6X9s have a unique mounting bolt pattern. I used these...
http://cgi.ebay.com/ebaymotors/6x9-R...spagenameZWDVW
Had to modify them slightly but it worked out well. Some people make their own out of MDF or whatever.