DON'T GET 6x9's!!!!!!! All it will do is cause bass cancellation, distortion, and the sub could damage the cone!
This sentance has me scratching my head to be honest. (no offence) First of all, don't you mean "don't get 6x9 SUBS" ? The confusion is that the way you worded it sounds like you're saying not to rplace the 6x9's period. As for the damage and cancellation, i can't see why i'd get cancellation as long as all speakers are in phase, and as far as damege, i don't see why anything i put back there would be damaged because for one thing the rears don't get very loud. Besides, i'd likely look for free air subs.
However, if you must get rear fills, get coaxial speakers, and place margarine tub underneath the cone (I kid not
ALSO, AMP YOUR FRONTS! Main priorety (if not done alreadY) you won;t get cancellation then!
Try amping front's before getting rear fills though..
No, i'm not going to mod anything. The stereo is fine as is except for the rear low end distortion, which i'd live with before i'd mod anything other than the rear 6x9 replacement.