id disconnect the dash speakers altogether and see how much sound you lose.
theres a couple of ways to do this but theyre all going to affect the sound adversely. you could get rca y splitters and run bot rca inputs off of the rear set of rca's, and then fade the h/u to the back some.... maybe move the 3.5's to the 4x10 slot in a home made bracket thus giving you a sort of center channel, and probably better staging...
i dont know as much about car audio as i used to, been alot of innovations since i messed with it heavily, but i can jury rig things like no other.