Rear Speakers

Hi i have a 2009 GL1800, this bike does not have any rear speakers fitted or wires either. i have also tried looking for the amp which i cannot find.
this is a GB model, i read in one thread somewhere that GB models do not have amps, the stereo does have front to rear fader, so i would assume that i could wire into the head unit if i knew where that was
what i would like to know is?
where can i plug speaker wires in so that i can run some rear speakers