The rear doors grille/hole for letting the sound out is way off axis with the speaker and a good percentage of the tweeter/midrange gets cut out because of the hole opening. Very piss poor design I might add. I added rear speakers and can barely hear them because of that. What a waste!
You could mount midrange/midbass speakers in the doors, and add tweeters on the doors aiming towards the center of the roof for the front speakers or such for the rears.
Best thing to do is to build kickpanels pods from the stock kickpanel locations for the best imagining possible, and in our case, rear speakers wont ruin any type of soundstage so put them in for the rear passengers.