My walls are the same and I managed to feed my rear surrounds by passing an old metal tape measure down behind the plaster board (took the right angled metal tip off it first). The tape came out behind the skirting which I'd removed, so I then used masking tape to stick the cable to the end of the tape measure and pulled it back up inside the wall. I was lucky that there was a gap between the dobs of adhesive. You might find you can do the same horizontally ( if I understood you correctly). If you are unlucky you may have to chase where you hit the adhesive, then patch it in after, like I did over my patio door to get to the last speaker.

Also scart leads are too thick for the connectors to fit, you might get away with HDMI and component.
See my thread (post number 12 if the link doesn't work right) to see what I mean:
KelvinS1965's setup.
Hope this helps.

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