Horribly stupid question but the manual says you press the ON/OFF knob (the volume control knob actually on the side of the Nav screen) and it turns the radio off. It says hold it down for 2 seconds if you want to shut the whole screen down. Well for me a quick press turns off not only the radio, but also the Nav screen leaving only a clock. Holding it down 2 seconds causes two beeps then the same thing, black screen with a clock. What am I doing wrong, is there some option I need to set? As it stands I only turn off music in my car by hitting the on wheel mute. ???