A device for indicating unauthorized use of a vehicle comprises a control unit (7) adapted to receive an access code and a display (4) for displaying the licence number (13) of the vehicle in response to control signals from the control unit (7). When the access code is not received, the display (4) shows an indication of theft, e.g. the word 'STOLEN'. In response to another control code, the display (4) acts as third brake light, thus integrating the theft prevention device and the third brake light. Preferably, the display (4) is installed between layers (1, 2) of the rear window of the vehicle and powered through the rear window heating (3).