Which model? Some have amplifier built into head unit, others have seperate amps. A number of shops repair the Becker's (
www.beckerautosound.com ) and LaJolla Audio. But most of us have replaced original units with aftermarket units like Alpine or Blaupunkt. Essentially, connections go bad between amp and speakers or between amps and head unit. Sometimes parts inside amps get loose due to solder fatigue.