I do not believe that the rear doors take speakers. I have not personally taken one apart, but I am sure you can customize something in there easily. All you need is something to cut the sheet metal.

Now, as for whether or not you should do it. Why do you want so many speakers. A 4 speaker system can get loud enough for people sitting in the car. And also remember the more speakers you have, the harder it will be to image them correctly. If I were you, I would spend the time, money, and effort into the set of speakers in the front and then the 6x9s that can do in the back.