Hi is it possible to install the rear interior light. When it wasn't factory fitted?

Yup. Some have done it. It takes quite abit of commitment though, cutting into the roof lining

Yes cut the lining and pull through cables to a power source - be aware it will not look like the factory one. Notice that the roof in the picture has an extrusion in which it holds the unit.

Does the cables for it are factory fitted? Do I need to take out the roof uphostery to fit it?

I don't think your Fabia will have the cables ready for it - no need to remove the roof lining.

You can fish the cables from the open cut to the front interior light.