Replacing rear speaker
Just unscrew 2 screws holding the grille to the rear deck, and then you can tilt and slide the grille out. Then, unscrew the corners of the speaker from the rear deck, and pull the speaker out.
I didn't notice if you'd changed the other speakers, but I'd recommend changing the dash speakers also. The front dash speakers come out in a similar manner to the rear deck speakers.
BTW, you can use 6.5" round coaxial speakers in the back without having to cut any metal, just have to trim the rear deck holes to fit the magnet spider. To go 6 x 9 flush mount does require some minor metal cutting, or if you use a speaker riser, you could probably avoid cutting metal.



