Aftermarket amp in 2005 Chev Cobalt

    I am trying to install an aftermarket amp and speakers into a 2005 chevy cobalt LS while retaining the factory head unit. It has the factory 7 speaker system that is powered by an external amp. The wiring diagrams that I have show that the outputs from the head unit are " ** Low Level Audio Signal" and " ** Low Level Audio (-)" where ** is the the speaker location. I have tried sending that "low level" signal directly into an amp however I get absolutly nothing out of the amp. I also tried sending the "low level" signal into a PSI converter then into the amp. Again absolutly no output. I do not want to use high level into my PSI Hi-Low converter, or install an aftermarket deck.
    Any help or ideas you may have would be very appreciated. The one idea I had is if perhaps they are running a balanced signal into the factory amp, and using chassis ground as signal ground. Anyone have any thoughts?

Thanks for the help,
Zenon Kazienko

Two things... External aftermarket amp you're using must have it's remote turn-on connected... and the other is, if you're going to use the output at the deck you'll have to plice rca's into that signal to feed right into the rca input of the actermarket amp. If you're going to use the high level output after the factory amp, then you'll have to connect them to the aftermarket high level input.