You really can't wire speakers in series without it sounding like crap. The reason is that speakers don't have a simple resistance. They have an impedance which means that the resistance changes based on the frequency. This is because each speaker's windings are basically coils. Coils can be used to filter out high frequencies for subs.
If you want to keep your full spectrum of frequencies, you will have to wire each speaker pair in parallel. Then on one side of each speaker pair, you could add a 2 ohm resistor. Make sure the resistor can handle at least 1/4 of the amps output power, or pick up a speaker rheostat from Radio Shack and adjust it to 2 ohms.
Personally, I would try it without and just lower the output to the rears with your fader control. The worst that can happen is you burn out a fuse in the head unit, or start a fire and burn down your car
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