I have a 1998 F-150 with the preumium Ford head unit & CD changer. The problem is that the left front speaker will randomly cut out. If I crank the volume it will come back on. This problem is getting worse over time. I did replace all four factory speakers with Polk Audio speakers about 3 years ago and have had no problems since then. Is the amplifier built into the factory head unit, or down the line somewhere? I have a Sony MD player that will control a CD changer as well. Does anybody make an adapter that will allow my Sony MD player to control the factory CD changer?

Thanks,
Jim