setting the rear volume of the radio of skoda fabia 2011 MT 1.2

The stereo may have a failed function called, "Fade" which puts more volume either to the front or rear speakers. Make sure you did not change the Fade to the front speakers by accident.

Thanks but the radio didn't come with FADE option.

Then the rear speaker circuit is burnt out in the stereo. You should still wiggle the connecters around to see if anything happens.