The rear grill covers snap in place. If I recall you push in at the base of the vertical side and then gently pull up, unsnaping the cover.
I think the speakers are 120mm, a very limited size for replacements. I replaced mine with 120mm Rockford Fosgate replacements ($100/pair). Sounds much better than stock but still lacks bass so I installed a tube style subwoofer. Hope this helps.

John H.