swap your speaker connectors on the back of your stereo (assuming its the factory becker), and see if the problem moves. if it stays put its probably the speaker.
wait, IS this a factory stereo? and its going through teh fader knob on the console? does the right REAR speaker cut out too or just the front? if its the factory setup, and the rear works, then its just the front speaker or the wiring. does jiggling the fader affect it any?