Thanks LugiaDark1! I suspected that I'd need an amp to accomplish what I was after. I only have a single 6.5 speaker and tweeter in each door, I don't have the rear door speaker. This is the reason why I wanted to throw some boxed speakers in the back. I could build a permanent structure back there, but figured it would be easier to buy some cheap boxes and use velcro to essentially mount them to the floor in the back. I'd use subs if I didn't have to mess with running an amp, but rather than do so I'd just probably just go with another pair of coaxial speakers.

Here is one more question, I mentioned that I have a single speaker and component tweeter in the doors. If I were to mount those in boxes in the back, could I just go with the 6.5 speaker and not mess with the tweeter and crossover (eliminate them), or am I required to use them? I'm not savvy on how components work. What I'm after is a little more bass coming from the back and then clearer sound coming from the doors up front.