You're looking at dumping good money into a bad location. Crutchfield seems to be saying 6.5's won't fit, so you're at 5.25's, and while the aftermarket deck helps you're looking at trying to overcome a lot. (small free-air speakers reflecting off the back glass)
I'd ghetto it up and get a pair of 6x9's in boxes.
Looking around, saw these:
http://www.amazon.com/Infinity-Kappa...nity+kappa+6x9
96dB sensitivity, crossover, for $100.
(e: The "ghetto it up" was a reference to the install, not the quality of those speakers. A pair of $20 wedge 6x9 boxes perched up on the rear shelf isn't exactly the most elegant of installs. It's just it gets the job done VERY well because it cleans up so many problems with the factory shelf location.)