Amplifier and Sub Wiring

I am tring to ad a set of subs to my factory stereo in my 99 Disco II. I Don't have the subs in the back door, I do have the six disc changer. If at all posible I would like to keep the original head unit. I was thinking that if there waqs factory wiring in the rear for the rear door sub I could just tap inot that for my remote power and my sound signal. In looking There is a plug that is caped there but i have not been able to find any power coming through the wires. I am tring to avoid running wires from the head because I do not have the metal peices to remove the stereo. Has anyone tried to ad a sub in the back before. i found some wiring for a Disco 1 that shows someone tapping into these wires but mine doesn't have the rear sub. Any suestions.