FYI - when I redid my entire system and had everything apart, I experimented with a number of things, being that I had all the panels off.
I was running a PDX 5, so sub and two channels to front door components,(Focal) but as I said, experimented with some stuff, so I made a custom mounting pod and fit 4" coaxials (Boston Acc.) behind the panel of each corner behind the seat, as well as put two 3.5" (Boston Acc) coaxials in a modified angular arrangement on a custom pod in behind where the grille for the center bolster is. All 4 rear speakers are wired in mono to the summed L& R rear channels of the PDX 5, and I taped over the tweeter, so not to produce any competing high end frequencies. I gave it lots of testing at various gain levels , and found that there was a very nice subtle effect when leaving them "just audible"....at lower volumes you can just hear them barely, but the stage is still very forward in the car. At louder volumes they are overpowered by the fronts totally. Again, it creates a bit of a fill effect, and I was very happy with the overall effect together with the fronts.
The funny thing is, that even though these speakers are small, they are pretty good quality speakers from Boston Acc. I have high passed them high enough to not get muddy at full volume. When I completely shut off the fronts (for fun) and listen to how the rears and centers sound alone, it is actually decent, really decent when still used with the subwoofer...anyway, figured I had the chance to try it while things were apart, so went for it....I really think it fills in the dead space nicely, albeit very subtle. I know this would not be the purist's stereo install, but was fun experimenting with it...I figured if it detracted, than they're out...but I found a slight fill and no sound quality detraction, so they are left in there.
Of course the center bolster is not cut in any way, and I have replacement corner panels that could replace the ones I cut to make grilles behind the seats.