I know WickedCAS had a 3-1/2" Hertz mid that supposedly would fit back there.
With regards to the entire four speaker debate, I believe hoodust nailed it in the head. Rear speakers will affect positioning (BTW, the lower the frequency, the more unidirectional the sound becomes), not necessarily cancel the fronts out. Given the same signal and no phase shift, there won't be cancellation. Wire the rears 180 out of phase with the fronts - now you're talking some cancellation.
I've heard some very good surround effects in sedans in the past, when done properly. The Levinson systems in Lexus cars use some proprietary surround processing and sound amazing.
So rears won't kill a system if competently installed. But unless there's some added effect which adds to the sound, I prefer just two fronts in the car.
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