I didn't see many good pictures of how this can be accomplished so I thought I would throw a few up to encourage others to try it. I'm skipping the whole process of getting to the panel that the speaker sits in, but you basically have to strip the rear of the seats and all plastic wall panels. Once you are there, pull and remove the black plastic speaker panel and remove the existing 6.75"-nothing-commercial-will-fit-here-easily speaker. I used the template that came with my Pioneer 6x9s to layout the hole I would need to cut in the panel. I traced it out and cut it with my jigsaw, so it looked like this:
I had to do a bit of grinding on some of the plastic ribs in the panel to make the speaker sit flush. I used the metal screw clips as you can see for tighter screw in than just through the plastic. Install the speaker and run the speaker wire, I soldered the old speaker plug onto the new 6x9 for a nice factory look. Here it is from the rear ready to install:
And with the panel reinstalled, speaker plugged in, tested and working:
There is plenty of depth in both directions for the 6x9s and my speaker covers fit back on and everything is stock and clean. There you go, easy!