The wiring is probably there but the radio wont support it.

 

so you can't buy speakers and make it work?

 

Take the panel off and see is the speaker wiring harness is there.

 

Zdejmij panel i sprawdź, czy jest tam wiązka przewodów głośnikowych.
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ale przez radio czy przez tylne drzwi?

 

Back door.

 

but someone wrote that even though there will be cables, the radio will not work with them

 

I did but you at least try. That is simple to do.

 

and if there is a bundle of cables, the radio won't necessarily support an additional speaker?

 

You could add additional speakers- but what are your requirements? Bigger sound, improved sound distribution, improved quality, bigger bass, louder?

 

What is the point? Is the sound system no good?

 

Additional speakers can be wired in parallel. Despite impedance halved, in practice ok.