there is sufficent depth for a 6.5 to fit, as long as you don't get something over 3.5" (estimate) in mounting depth, but you can deal with probably up to 4"of depth via mounting rings/baffles of whatever thickness you need. I used a 1/2" thick plastic cutting board from wally world, cut the appropriate size circle/hole for the speaker, mounted the ring to the oem speaker mount, then mounted my 6.5's to this.
No i did not replace the rear speakers, they are so far away from the front I saw no real reason. I usually fade the sound most of the way to front when I'm driving it anyway. If I can find some good sounding cheap co-ax's I might, but it's doubtful. The 5 is the wife's car, and she didn't want me hacking it up except to install the new comps since the oem whizzers s*ck so bad. I upgraded the speakers in my Mz3 last year with the same one's I installed in the Mz5 as no amp is really necessary unless you like it really LOUD!
