I had that problem in December, it was the heater resistor. It's £99 from your local Renault Dealer, I didn't have time to look online. Another issue could be that the brushes in the motor are goosed,which in Renault speak would mean a new motor,but if you can get the motor out a domestic appliance engineer will generally be able to sort you out with replacement carbon brushes.
In my case though it was the resisitor.If you pop the bonnet, lift the cover from around the drivers side wiper,underneath you will see a wire loom going from left to right into a black connector to the left of the wiper.That is the heater resistor, there are two bolts (6mm I think).Speak nicely to the parts bloke and he may let you plug the new one in first to see if it works or not.It did in my case.
Hope this helps