You're looking for a grey/blue wire with a plug on it back there somewhere, I'm 90% sure I had one hangin about back there when I had my fusebox off. If you find it, check it with a multimeter/test light to make sure it gets 12v with the lights on!
The other thing you could do is chop the grey/blue wire off the stereo ISO plug, as no modern stereos use this feed anymore.