Alright Jon
As it happens i did this exact thing on my mates mk1 clio a few years back.
Basically you find the brake wire in the wiring loom that connects up the rear light cluster. You can use a multimeter to find out which one is the brake light wire (i.e. check for a current when someone puts the brake pedal down). Once you have found the correct wire, get a block which allows you to connect one wire to another one(cant remember what they are called, they are normally blue, open in half and have a ridge for the original wire to run through and one for the new one, there is then a metal spicey thin that comes down to cut the sheath surrounding each wire and make the connection)
Once you have done that hook the earth up to the eachthing point(seem to remember its located really close to the rear clusters,maybe behind them) and then it should work.
HTH