you should find it next to the driver's side rear light (behind the carpet); mine was hooked onto a bracket, you just have to slide it upwards to get it free. i tugged at it for ages before realising this lol
oh, and it's one of those white plugs; it'll be the one with four pins and its the only one that the shelf plug will fit into - there should be a couple of connectors that are insulated (ignore these) and there was a two pin plug that was tied back on mine (not too sure where that one goes). it isn't that one either.
one last thing, i managed to get my sound problem figured out. My brother (used to be an electrician) used a meter to check the loom which was working fine; turns out my ageing stereo is the culprit. So if you get no sound it might be that.