If your car has rear spekers the disconnect them and simply wire right into the connector that goes to the stereo. That's wat I did in my punto, wether or not a seicento is the same I don't know
the stereo should have a plug at the back. the two green, two grey, two purple and two white wire are all for your speakers. using your head units fader and balance you can find out which wires are for which speaker.
firstly knowing you haven't go wires for the rear, you need to put some in. once the wires have been ran from the rear speakers you can begin to connect them to the head unit.