If you punch the gas at idle does it do it? How about with all of the elctronics on (rear defroster, radio, lights, wipers, etc), A/C and wheel turning at idle? It also could be a belt that just worn in. Just an idea. Thought i would add to the list. Is it a whining or a squealing.
Yes there is a way to add to a stock system. The only problem is that it will pick up that distortion that the stereo is putting out and amplify it further. But the positive side is that bass distortion is not as bad as full sound distortion. You will need a line output converter to convert from the high levels at the back speakers to low lever rcs signal input for the amplifier.