On my '98 Boxster, I am just now noticing that the backlighting on the radio, when it's on, only illuminates when the vehicle light switch is in either the "Parking Lights" or "Low Beam/High Beam" position. When the light switch is in any other position, and the radio is on, there is no backlighting. Is this correct? I would have thought the backlighting would be on any time the radio was on. I couldn't find an answer in either the Owner's Manual or Radio Guide.