Necvox rear seat screen (FD6569) interference

most car radios have an interference power filter, consisting of a special arrangement of a coil and capacitor. It is packaged in a plastic enclosure about 2 inches by 1 inch by inch. This filter is designed to reduce the interference caused by the system wiring picking up RF noise from the engine controls. In your case, when the switch is on, your video device (which can produce much RF noise) is interfering with the radio. Have the radio checked for the presence of this radio filter, and if it exists, try using two filters together (can be purchased at Radio Shack) wired in series.