Most 6.5 inch speakers require you to completely cut out the back of the speaker bracket to fit. Basicall the only thing left should be the ring to mount the speakers. This can be easily accomplished using a Dremel or a hacksaw...though a Dremel is a lot more fun.
But even after doing that, the speakers may still be to deep. Then you will want to use some longer screws and something to space them with. To protect you speakers from the elements I highly recommend some XTC Speaker Baffles from Crutchfield. Just make sure you get then narrow ones.
I've never tried mounting 6x9 speakers in the rear so someone else might want to chime in.