I changed the front woofers to Pioneer units, and ditched the rears completely, replacing them both with a dual coil sub and running the feeds directly to this.
Made a bit of an improvement, but ultimately didn't sound great until I ditched the standard stereo and replaced with a Sony HU & changer.
Now I've got a Sony flip-front Casette HU, Sony 6 CD Changer, and connectors to run the dash display & steering wheel controls - all 2nd hand off ebay, and change from £100!
Silly not to at those prices really....
I've still got the original Renault HU & CD Changer to put back in the car when I eventually come to sell it!