A big THANKS to Memphis Man for the photos above. I was able to remove the speaker grilles without removing the shelf, saving a major amount of time on this job! Now here's the best part. Thanks to his photos, I was able to pinpoint where the tabs were and rather than use a screwdriver, I placed my thumbs over where the estimated spots where the tabs are located, and gave it a quick push and the front tabs popped right up. No tools needed!
Now you may be able to do this without removing the back seat which would really be ideal but I wouldn't recommend that as you need the room to make a lateral push to avoid the potential of damaging the grills. I had other repair work needing to be done on the rear seatback so that came with the job.
As far as the remote brain for the door & trunk locks, I have abandoned that job. I'll just use my keys. My Dad lost one of the remotes and my brother lost the other, so the next owner can deal with that.
My RetroSound speakers are in and properly installed, and my rear seatback is repaired as best it can be (see the Wanted section).