There is a previous thread on this. About 2 months ago i described how to do it. But here goes anyway.
Find your fan switch, its in the radiaitor on the right hand side about a 3rd of the way up (a clio 16v rad anyway, not sure which rad youre running). Splice into each wire, of which there are 2 and extend the extra wires you have just added to wherever you want the switch. Dont forget to include the original renault wiring in the new splices or your fan switch wont work.
Get a Single Pole Single Throw toggle switch with a 10-15 Amp raiting and connect your two wires to the back of it.
Now when you flick the switch, the fan cuts in. It will also run from the fan switch as normal.
However, dont forget to switch off the fan when you leave the car. You could use a switched ignition feed for the +12v, this would mean the fan only works when ignitions on.
Hope this helps...