Wires for rear speakers

[komichal]

Greetings!
I have 2007 Yaris II 5-door hatchback.

Music from the radio was too quiet on the back seats and I found out that I have no speakers in the rear doors installed.
The hole for the speaker is there in the door but it has never been used, it is still covered by the plastic.

Buying and mounting the speakers is simple but how about the wiring? At first notice I was not able to see a cable that I would be able to connect the speakers to. But maybe it is somewhere well hidden?

Or do I need to do the wiring on my own? That is something that will probably completely stop me from installing the speakers.

Thank you for sharing your experience.