If it's an original Renault part, marketed as an optional accessory, there should be a wiring loom for it already in the car.
If you bought it from a Renault dealer, they would know whether it's got a loom or not, based on year.
If it's only marketed as a replacement part, and not as an optional accessory for retrofitting, there are slightly higher chances of a loom not being present.
A Scenic owner who has this accessory installed would be able to provide a better answer than me as to where the wiring loom should be coming out,
but I have a couple of educated guesses:
1) Beneath the floor carpeting under the center passenger seat, i.e. underneath where the manual says the socket should be installed.
2) Hanging down from underneath the center seat cushion.