hello there, i had to install a similar system into my missus 07 307 sport, i did it using the original headunit which had no pre-out as i didnt want to start destroying the originality of the car as at the time i didnt wanna invalidate the electrical waranty on the car, i bought 4 high power to low power rca converters, they connect to your normal speaker wire outputs on the headunit, i replaced all 4 door speakers with vibes and hooked a 4 channel amp up under the passenger seat, i had to put some inline supressors in due to the amount of electrical interference on these peugeots, i then put in some rca splitters off the rear speakers and run em through to the boot and into another amp which i used for the sub, you dont have as much control from the headunit as most people do like but you can offest the gains through the amps individually to get the sound your looking for, the setup the wife now has makes your eyes go blurry with the bass when you crank it up and the speakers dont screach at you as though your ears are about to burst but all in all its a pretty good system.
cheers M