I thought about doing it on my last Leon, but couldn't be bothered, so I wrote it off and (eventually) bought a new one instead.
AFAIAA it is fairly straightforward, just hassle having to feed the cable through. Get your head behind the rear wheel and you can see where it sits, just have to pull the claw forward and pull the ball at the end of the cable out. In car it is attached literally behind the handbrake, pop the back of the centre console by the rear ashtray off and you can see it.
Would suggest whether you try and disconnect both ends, then attach the new cable to the old one and pull it through. Don't know which way is best to pull though, sorry!