like Ogre said, they are pre wired, you just have to swap the speakers. They are 6.5 speakers not 6x9s.
Waisay why do you want to change the rear speakers? why not invest on the front ones?
Front speakers are the ones which you will notice the most in terms of sound quality improvement.
But still if you want to... IMO Infinity Reference 6.5 coaxial or Alpine Type R coaxial are nice speakers
OR
IMO what should be done is, you shift your front speakers to the rear and get some good speakers or comps for the front.