I put 6.75" Coax in my rears this summer. I will add that if you can remove the rear panels entirely, it makes the process SIGNIFICANTLY easier. The right side was an absolute pain, due to the limited "holes" in the aft cladding panels. The left side has a fairly big hole where the tire-jack fits, but the right side does not.

I experimented with clipping out the webbing on the back of the speaker grilles, and I highly recommend you clip most of it while you're in there. Not all of it, mind you, just the parts that stick out the most. There should be enough "flush-fit" areas left to hold the stock cloth grille cover in place.

I recommend COPIOUS amounts of weather-stripping/padding to help prevent any rattles from a larger speaker being installed. Dynamat will probably accomplish the same thing, if you choose to install it on the sides of the vehicle during a speaker swap. Initial test-fit indicated that the mount depth was too deep, but I managed to compensate for this by stacking two pairs of trim rings under the mount lip of the speaker and sandwiching them with spacers. This raised the mount depth enough to work--and still was shallow enough to clear the grille with the webbing clipped. (I included this info in another thread here).

I'm curious if anyone has done 6x9's in the rear. I'm almost certain that it could be done, but I wasn't 100% sure I had the expertise to do it.