I am almost 100% sure there is only one circular hole in the rear deck. Many have put a free air sub in that location.
Some people will not even run rear door speakers because they want the sound stage to be coming all from in front of them. I have tried it both ways and not running the rears still gave a great sound but didn't have the loudness I had from all the speakers running ( which make sense two less speakers = not as loud)
as for adding speakers to the rear deck I say go for it if you want, it will help the sound when you have all windows open because the sound will bounce off the rear window, while the door speakers may become over powered by the wind noise from the open window.
There was a night and day difference when I added my component set up from and got the tweeters in front and above the window.
Hope this helped a little...
