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Paul, thanks again for your responses. I am looking for better sound quality.
Since the wires of the stock amp have been cut by the previous owner, I am wondering if it is worth the effort in figuring out the wiring and re-connecting the stock amp for the rear door speakers.
It somebody had experience with it and thinks the stock amp is of lower sound quality than the head unit output, then I will probably not spend the time in making the stock amp work again. However, if the opposite is true then I am willing to spend the time.