i was recently given a 98 gmc jimmy by my dad who had 2 cars and was moving to hawaii . the guy he bought it from had a aftermarket cd changer instaled but it has never worked since we have owned the car but the stock radio did. so when the battery suddenly had a drain i took out the changer and control deck for it. now the stock radio comes on & shows radio stations but no sound comes out of any speakers . any ideas where 2 start