A spatial position of at least one first body part, arranged within a sensing range of a sensing device of the motor vehicle, of a vehicle occupant is sensed with respect to the motor vehicle by the sensing device. An anthropometric data model is provided with data including the dimensions of predetermined body parts of a person and the positioning of the body parts with respect to one another. As a result, a spatial position of at least one second body part of the same vehicle occupant which is arranged outside the sensing range of the sensing device can be determined, taking into consideration the sensed spatial position of the first body part and the anthropometric data model. At least one functional unit of the motor vehicle is actuated taking into consideration the sensed spatial position of the second body part.