An assembly for forming an air-guiding element for a motor vehicle, comprising a shell-shaped air-guiding body and a brake lamp. The air-guiding body has an elongated opening which extends through the air-guiding body. To form an air-guiding element, the elongated opening can be closed off at least in regions by a light emitting surface of the brake lamp. By disposing the brake lamp in a predetermined position, the brake lamp is enclosed at least in regions by the air-guiding body. At least one retention element is integrally formed on a retention side of an air-guiding body portion of the air-guiding body. At least one counterpart retention element is integrally formed on a retention side of the brake lamp. The retention element can be retained on the counterpart retention element so as to retain the air-guiding body on the brake lamp in the predetermined brake lamp position. In the connected state of the air-guiding element and the motor vehicle, the air-guiding body portion is arranged below the brake lamp. The retention side of the air-guiding body portion faces toward the retention side of the brake lamp.