Ok, it probably is vacuum, but before you change out the control in your dash, PLEASE see if there is a check valve coming from your intake manifold that provides the vacuum to the system. This is a REAL EASY fix that can drive you nuts. If your air works fine at idle and then defaults to the defrost vents under acceleration then that is the problem. The valve is usually black and is found in line just coming out of the intake manifold for vacuum. This is a one way check valve and you can check it by sucking in and blowing out. You should be able to do one but not the other. If it's constantly blowing from the defrost vents then it's probably the vacuum control switch which I believe you have to replace as a whole unit. Hope this helps, Paul.

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