There is more info on the subject of changing speakers in the below site. It is in norwegian but with pictures to make it somewhat understandable... :)
http://elbilforum.no/forum/index.php/topic,5870.0.html
Good luck!
I just bought a new Peugeot iOn and quickly saw the opportunity to fit rear speakers in order to improve the sound quality. My stereo system is of the standard type with Bluetooth and USB input. There was a fader function in the menu, but I did not know if the speaker wires were pre-installed...