A demand for a change in the operating condition of at least one of a plurality of vehicle mounted devices is issued by means of a single push button switch attached to the center of the steering wheel. When the vehicle occupant intends to manipulate a device such as a radio, stereo unit or an air conditioner, he depresses the switch to supply a microcomputer with a demand signal. The microcomputer produces an instruction signal with which a sound synthesizer produces sequential vocal announcements which are emitted via a speaker. The occupant depresses the switch again within a predetermined period of time after the emission of a vocal announcement naming a device intended to control so that the sound synthesizer now provides various manipulative information relating to the available manipulations of the selected device. The driver depresses the switch to issue responsive instructions in response to an announcement of desired manipulation. The microcomputer thus operates to produce necessary control signals which are fed to a corresponding control unit of the selected device. A sound recognizer may be used in place of or together with the switch to detect the demand and responsive instructions from the driver.