I have a problem with my 2004 Impala windshield defroster. When I turn the selector to defrost I do not have any air coming out of the defroster windshield vent, but I do have air coming out of the side window vents. The blower is blowing but air is not coming out of the defroster vents, but does come out of the dash vents and the heater floor vents. Another thing I noticed if I set the control to blow out the dash vents it does blow out, but when I move the selector up one notch to the dash/floor position the air now blows out even faster from the dash vents. Anyone know whatis causing thisproblem?

I should note that I just had work done on my impala due to a leaking intake gasket causing oil to get into the coolant. The shop replaced intake/valve covergaskets, replaced thermostat, flushed cooling and fuel systems, new plugs, oil and coolant. I had not noticed this problem prior to this repair, so I'm wondering if the problem might be related to this repair. I don't know if the heater controls are electric or are vacuum controlled. I did check in the engine compartment for any loose vacuum hoses or wires and I did not see anything out of place. Any help with this issue will be greatly appreciated. Thanks.