Abstract: A unit (1) for use in a vehicle is interrogated and identified by a fixed interrogator (2). A wireless form of communication is established between the unit (1) and interrogator (2) to permit transfer of data to the unit (1). As well as appropriate communication circuitry (12) and an identification store (19), the unit (1) includes processing of the incoming data by a microprocessor (32) to provide an audio signal for energizing a loud speaker (36) in the unit (1) to provide a sound output for the vehicle occupants. The unit is powered by internal batteries (22, 38) to be usable even when the vehicle ignition is turned off. The unit (1) is made self-contained to be mounted wherever convenient. The principle can be extended to providing data to control a visual display in the unit (1). The unit (1) may be provided with a user-interactive input such as a keypad (48). A radio link is described but other wireless means of communication are feasible.