xD Mike

Hello, Well basically I am trying to install a subwoofer with an aftermarket amp in a 2010 Fusion with Navigation and the Sony Sound System while leaving the factory amp installed. I searched the site for a good hour and couldn't find a definitive answer on what to do. I have tried using a line out converter and attaching it to the left rear and right rear door speaker wire from behind the navigation unit(head unit) and also tried connecting it to the left rear and right rear door speakers from the center plug on the factory amplifier. I tapped into the remote turn-on wire that is plugged into the factory amp and that works fine. When I try the line out converter it just does not work. I have also tried using the high-level input wire that came with the aftermarket amp and that did not work either. The aftermarket amplifier will turn on fine after running the power, ground, and turn-on lead, but no sound will play through the sub. Is there any advice you can give to help us get this working? Thank you.