An air guide is assembled on a structural component of a motor vehicle by at least two fasteners that each include a first component and a second component. The fasteners project laterally from a frame defined by lateral edges of the structural component in a direction substantially perpendicular to the fastening direction. The first component is joined to the air guide by a retainer that breaks following an impact directed substantially parallel to the fastening direction. Lateral edges of the air guide exhibit guide parts situated substantially opposite second components. Each guide part guides a lateral edge of the structural component supporting a second component inside the air guide following a breakage of the retainer of the first component working together with the second component, the breakage resulting in movement of the air guide towards the structural component in a direction substantially parallel to the fastening direction.