I think it depends on whether you're rear seats fold down or not.
Mine do so it was a simple case of disconnecting the rear brake light, popping up the rear deck and sliding it forward and out. Not sure what all the other removal stuff was about as I didn't have to remove anythig other than the deck cover itself.
Can't see how standard speakers can be screwed into the OE locations without an adapter as the profile where the speaker meets the rear deck is not flat (it goes up & down thus leaving air gaps all round the speaker).
I used Pioneer custom fit units, after trying to make adapters, although you can now buy adapter rings for the rears now.