In my 1996 GMC Sonoma all of the air flow is directed to the defroster and the floor vents no matter what is selected. A/C, heat or just vent I cannot get air to come out of the dash vents. When at idle there is a slight airflow out of the dash but at speed all of the air is directed to defrost and floor. Is ther some linkage adjustment behind the dash to look at? Any help herer would be greatly appreciated.