Or you could buy an amp with high level input which takes a spearker level input rather than an RCA input. To wire the amp you connect the ground and power as usual, then on the back of the head unit do the following:
With a multimeter or circuit tester, find the wire that gives a signal (12v) when the key is turned (engine does not need to be running), but gives no signal when the key is at the off position, this is your remote wire. Splice into this with the supplied remote wire and connect it to the amp.
Find your speaker wire. I used the rear speakers. There is a set of 8 wires in the other block next to the power cable (from the previous step). They are LEFT REAR, RIGHT REAR, LEFT FRONT and RIGHT FRONT. There are 8 wires because there is a positive and negative for each channel. On saying that you are going to use the rear speakers, you need to splice into LEFT REAR + and RIGHT REAR - so that you get the bass from both channels merged into 1.
Does that all make sense? Id be happy to explain it in more detail if need be.
I did this on my first Clio (2000) and it sounded really nice.