Abstract: The air conditioning installation has air treatment and distribution means in a housing with air outlets on an upper face. De-icing and central aeration ducts are in an oblong diffusion unit above the upper face of the housing and under the upper fascia of the dashboard. This upper fascia has a diffusing surface in excess of 1.5 dm2 and preferably in excess of 2.5 dm2. In an embodiment, the diffusing surface is produced by multiple perforations.