hi, i have similar surcumstances with my clio, being the authentique it only has front speakers, there are two main plugs on the head unit, one is power, and the other is speakers, (ignore blue scribble) the main socket with 8 pins on is the speakers, going left to right top to bottom 1-8 they are paired 1-5 2-6 3-7 4-8 and as far as i can remember

the outer pairs are the front and the inner pairs are the rears, if you run two (twin core) cables down passenger side sill, then take one cable across infront of rear seats, as this is where the cars wiring loom runs, if it dont have the speakers in, then renault, saving money dont but the cables in
i hope this helps