You can't simply 'bridge' speaker wire to run another set of speakers. When you go to a concert do you have your back to the stage? This is why it is better to invest in a decent component set-up at the from and a sub in the boot. You then cover the frequency range (tops, mids and lows).
Connect the fronts and the 6x9s, leave the rears. Daisy chaining the speakers will sound rubbish.