Bartoszek1998 wrote: And where are the cables more or less bursting at?
Open the back door and you will see a flexible pipe filled with cables.
Most often, the cables break / break at the bends in the bundles running to the rear door.
They power not only the loudspeakers, but also the electric locks and power windows (if any).
But...
If there are no problems with closing / opening doors and windows and only the rear door speakers are not working then look elsewhere.
Maybe the easiest way is to disconnect the speakers from the radio and check it by connecting a battery, e.g. 9V, to the pins of the speaker plug.
The characteristic "knock" on each speaker in turn means that the installation / speakers are working properly. There are 5 speakers (3 front and 2 rear).