The slider 31 is preferably integrally formed at the BMS plate 30 by plastic injection molding. A gradient structure 32 inclined to have a gradually increasing thickness in a direction opposite to the inserting direction is formed in a portion of the slider 31. The gradient structure 32 is for pressing the slider 31 into the coupling groove 22 and prevents the BMS plate 30 from being separated from the air duct 20 due to external vibration or impact or being damaged by shaking.

Since the guide rib 23 forming the coupling groove 22 is made of aluminum material integrally with the air duct 20, it is preferable that the slider 31 is made of plastic material and tightly inserted into the coupling groove 22. It is preferable that the gradient structure 32 is located close to a rear portion of the slider 31 so that the slider 31 is tightly fitted into the coupling groove 22 and simultaneously the BMS plate 30 is completely inserted.

In order to fix the position of the BMS plate 30 assembled in a sliding manner more firmly, a stopper 33 having a hook structure and made of plastic material is provided on a part of the BMS plate 30. The stopper 33 may be integrally formed when the BMS plate 30 is formed. In addition, a fixing plate 34 having a hole or groove form corresponding to the stopper 33 is provided at the air duct 20 or a predetermined support plate located under the BMS plate 30. When the slider 31 is completely inserted, the stopper 33 is hooked to the fixing portion 34 to effectively prevent movement of the BMS plate 30.