New here, so I apologize if this subject has been breached at another location. A search of the forum couldn't find anything. My 2015 GMC Terrain SLT will blow air through the defrost vents, and through the floor vents but will not blow from the front vents. A Google search suggests something called a 'Blend Door Actuator'. What is that, and is that the problem or am I missing something else? Thanks.