The device (10) has a light (18) e.g. LED arrangement and incandescent lamp, connected with a connection (20) at grounds (26, 28) of an on-board electrical system (24) of a towing vehicle (12), and another connection (22) at a line (30) for supply of electricity from the electrical system to the light. A constant current source (34) has connections (36, 38), which are connected to the electrical system with respect to the electricity, and to the line for the light and to a voltage measuring circuit (32), respectively.
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