I have heard that the two speaker next to the rear seats inside the car are the subs. I am putting in a whole system with a 12 inch sub which will go in the trunk/ Can i take out the interior 6.5 subs and put in midranges since the stock subs are crappy?

 

There's no reason why you can't. Go for it.

 

Yeah dude go for it. I'd recommend that you go with some dedicated woofers though as opposed to mid-ranges. That way you can keep your sound stage up front. Or least low pass them in the 200-250 range.