I have a delco radio in a farm tractor early 2000's. Radio worked yesterday when someone else was operating tractor and listening to radio. Today radio has power, volume numbers go up and down as volume is changed, it will say mute, can adjust time and bass, left, right, etc., but no radio channels or bands can be gotten. When radio is initially turned on, screen shows "Ex Audio" for a few seconds. Did a button get incorrectly pushed? Antenna problem? What do I do to hear the radio? Thanks.