A motor vehicle (1) has high-frequency transceiver stations (3a, 3b, 3c) for access control to functions of said motor vehicle (1). A portable access device (6a; 6b) is designed to exchange data with the high-frequency transceiver stations (3a, 3b, 3c), and at least one sensor device (5) detecting activation is arranged on the vehicle. When the sensor device (5) is activated, a signal connection (7a, 7b, 7c; 8a, 8b, 8c) is established between the radio-frequency transceiver stations (3a, 3b, 3c) and the portable access devices (6a; 6b). In this, a signal parameter of the signal connection is detected and is stored as a reference signal parameter. During subsequent signal connections, the current value of the signal parameter is determined, and a position of the portable access device (6a; 6b) is determined on the basis of the value of the further signal parameter and as a function of the reference signal parameter. Subsequently, a function of the motor vehicle (1) is enabled or disabled depending on the position.