Yes, I think that is the best place for the rear speakers. The backplate is thin and would expose the back of the speakers to different objects from boot. In fact is advisable that the back of a speaker to be separated, as much as possible from the front of the speaker. Secondly, for the people from the back sits would be very unpleasant to have the speakers near their heads.Hm, do you think this is a good place for sound? Isn`t it better on the backplate for the frequences....Yes there is only little place in this car, but I think there the sound will run into the universe of the backfootroom. May be I have to listen to your car to hear the opposit...My theory says no.
I showed you my installation just as an alternative, I think that is the best place for rear speakers for me, but anyone can do what they think that is better for them.