One day the radio refused to switch on whilst I was driving, however, 10 minutes into the journey it came on alone. When I parked and switched off the car, although the display of the radio was off, the on/of radio button light was on and would not come off even when I locked the car. This has drained the battery. Any ideas what this could be? This faint light on the knob will only go off if I disconnect the battery.