Just came across this by accident - I'm adding heated seats to my Mégane at the mo and thought I would share what I know.
Everything is there, the wires, the plugs, the fuses, all that is missing is the actual heated element in the seat - which I am ordering from Renault next week.
This is very easy to fit, the seat cover simply lifts back. If you look under each front seat, there is even a connector there waiting to be plugged into the element!
Basically, all you need is the elements for both seats, two switches, and new trim for round the handbrake with holes for the switches. I'll keep you posted on the cost of the elements, I know the switches are £9.90 each, and the trim £17.22.
I had a little play around with the plugs for the switches today using both ends of a paperclip to create a complete circuit. You can hear the switch activating and you even get a "HEATED SEAT ACTIVATED" message on the trip computer.
And to think I went through two winters on cold leather without knowing how little needs done!!

Ottoman Blue 2.0 VVT Privilège 5 Door - Full beige leather, ESP, Xenon headlights, Carminat Navigation, Cruise etc.
Ink Blue 2.0 VVT Dynamique 3 Door... Black Leather, heated seats, panoramic sunroof, tyre pressure sensors, Hands Free etc.