speaker/amp problem

I have a 96 explorer Eddie bauer with the JBL system and one of my back door speakers has a fuzzy or static sound in it that comes and goes quite often. I have changed the speakers thinking that one was blowen but that didnt help. so now I am thinking that it could be a channel on my amp since none of the others are doing it. could someone help me figure this out

the factory stereo is almost 10 years old .... time to replace it