I can barely hear the 2 speakers behind the seats. I have the plain stereo system. I assumed that they where small subs, but they don't provide any bass. They barely make any sound at all. I tried moving the sound to the back a little, but then the fronts got quiet and the backs still didn't help much.

 

the backs are AWFUL. they are not subs, just really pathetic full-range speakers. if you want a good upgrade for fairly cheap, get a good set of front components and fade the sound all the way foward...

 

I would first take a look at your fader control- sound to me (no pun) that you have the setting too much to the front
Try rear fader to +2 and right balance to +1 or +2 then bass +2
good luck

 

I have the fader on 0. The rear speakers just don't do anything.