06 Ford 500 aftermarket amp?

I have a 2006 Ford Five Hundred SEL with the factory 6 disc mp3 player with no factory amp.  I am trying to install a MTX 6500d with 2 MTX 12's.  I have the basic connections hooked up (power, ground and sub)and have one of the boxes built and installed.  I am having problems getting the amp to "read" the speaker inputs.  I found the rear speaker wires from the factory head unit by removing the rear door panels and tapping into those wires.  I ran them back and hooked them to a line level converter.....no sound from the sub.  I tested the wires that I hooked to for voltage.  Both wires had +5.81 volts.  Shouldn't one of the wires be -5.81 volts?  Anyway I thought that I could hook these wires into the high level inputs on the amp....no sound from the sub.  I am having i hard time understanding how this is going to connect to the factory headunit.  If I hook a speaker to the wires that I tapped from the headunit it works.  But when I hook it to the amp it doesn't.  All help is appreciated.

Thanks,Mike