Does any one know how much it would cost to repair my car stereo & install ONSTAR FMV ON A 2004 CHEVY IMPALA?

-I HAVE A 2004 CHEVY IMPALA... THE PREVIOUS OWNER BLEW THE FACTORY AMP, AND DECIDED NOT TO REPLACE THE STEREO.. THEY THEN DECIDED TO DISCONNECT THE AMP AND RUN THE CABLES DIRECTLY TO THE FACTORY STEREO.. SO NOW I BASICALLY HAVE THE FACTORY STEREO, BUT WIRES RUNNING DIRECTLY TO THE STEREO... I CANT FADE MY SPEAKERS FROM LEFT TO RIGHT OR BACK TO FRONT..I KNOW NOTHING ABOUT STEREOS, OR CARS FOR THAT FACT... THE CAR WAS A GIFT, AND I WOULD LIKE TO ADD ONSTAR FMV TO IT.. DOES ANYONE KNOW IF THIS IS GOING TO BE A ISSUE, O AND THERE WAS ALSO A AFTER MARKET REMOTE START INSTALLED (WRONG MIGHT I ADD).. IS THAT GOING TO BE AN ISSUE WITH GETTING ONSTAR? PLEASE HELP ME OUT, AND SERIOUS REPLIES ONLY.. IF SOME ONE IS IN THE LAKE COUNTY INDIANA AREA AND KNOW OF A PLACE THE SPECIALIZES IN THIS, IT WOULD GREATLY HELP.

THANKS A BUNCH:)Your best bet is to get an aftermarket stereo, and adding onstar to be integrated into your stereo and car is VERY expensive