Well Markus you leave out some important information but on my old GOLF when I needed to do this I prised the cover from the Grab Handle and undid a couple of hidden screws - then I had to crouch down really, really low and remove three screws at the very bottom of the interior trim panel. Then a sharp upwards blow released the panel - as far as I remember.
Of course - your car may not be a Golf and it may be different - but not that much different I suspect.
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