Here it is. I looked up the info on Crutchfield. You can view all possible speakers/headunits available for your car by
clicking here
Here's some quick info right away about speaker sizes that fit your car...
Factory replacement info:
In-dash stereo
7-1/16" depth; Size code E,S,U
Front speakers
Kick Panel: 5-1/4"
Rear speakers
Rear Deck: 4"
6x9's will not fit on the back deck without modification to the metalwork underneath the trim panel itself. Because of this, I do not recommend doing it. I also personally am not a huge fan of 6x9's in general because I do not like the quality of sound they produce. I almost feel your best bet would be to buy a really good set of front speakers (because the front sound stage is the most important when it comes to properly imaging sound) because the rear fill is not as important as you would think.
I think you'd be much better off getting a 10" sub for some bass (because I believe that's why you want the 6x9's) and removing the speakers empty in the back deck and leaving the holes empty. That way the bass from the sub in the trunk can come through to the inside of the car. Just a thought.