Apparatus for causing an external light system of a motor vehicle to be actuated when the vehicle engine is running, the vehicle having an electrical battery associated with the engine, and the running of the engine causing an AC signal to appear with a DC component at a terminal of the battery. The apparatus has an electrical circuit with a filter for filtering the DC component from the AC signal to provide a filtered AC signal indicating an engine running condition. The filtered AC signal or other engine running signal obtained from the filtered AC signal is compared with a predetermined value and an actuating signal is provided when the engine running signal is above the predetermined value. An actuator responsive to receipt of the actuating signal causes the external light system of the vehicle to be turned on.