Just an update on this, I purchased the PAC mentioned here for non amplified stock radio. Plugged PAC into stereo, then PAC out to Lockpick, then Lockpick out to stock harness / speakers. PAC has front and rear RCA outputs. I ran rear outputs to my Rockford AMP and wired AMP to my MTX subs. Everything works flawlessly and sounds amazing. I also added a gain knob so that I can control the subwoofers output from the front seat. I was concerned about all of the piggybacking, however it did not seem to cause any issues at all. All that being said, if I had not loved the look of the stock NAV unit, it would have been much easier and about the same price to have just changed out the head unit to an aftermarket double din. Lockpick 225.00 PAC 45.00 sub gain 10.00 so total 280.00 and a lot of piggybacking and rigging to do what an aftermarket stereo for around 300 would have done. All and all I am very happy with how it all came out but I am not sure it would be the right option for everybody. Good luck to anyone doing this and feel free to ask if you have any questions.