How do you repair blower motor on Renault Scenic 1.4 16v and what are the most common reasons for this part to stop working?

The blower motor can stop for several reasons.
1 Fuse
2 Relay
3 Switches
4 Resistor Pack
5 Motor
If you can get 1 (Usually fastest) or more speeds then suspect the Resistor pack.
Nothing at all resulted in it being the motor on ours (£65 from PMA) but check the fuse first and replace if necessary
The easiest way to find out is to remove the glove box you can then reach the plug on the motor (Red & Black wires). Bend down in the footwell and look up and to the centre of the car, plug is behind crossmember but easy to reach and remove.
Test the wiring with a volt meter or test lamp if everything else is ok you will get around 12.1 volts when switch is in positions '1 to 4' and nothing when on '0'.
You can also see and feel the resistor pack by removing the trim around the electronic dash - I put a small digital camera in there and then had a look at the pictures on the computer. When the resistors fail they very very often damage the plug and wiring (Renault supply an upgraded part and loom). It is possible to replace it through the opening but you need to yank on the motor wiring to get some slack and very small hands.
I changed my motor ok by removing all the lower facias and then undoing the top facia and moving it into the car resting it on top of the steering wheel (Adjusted fully towards you and fully down). There are lots of screws and wires that need disconnecting so it's best to use a Haynes Manual but ignore removing the steering wheel part.