heater and ac will only come out of defrost vent! on 1990 Mercury Cougar

AT FIRST THE AIR WAS COMING OUT OF THE PROPER VENTS! WHEN USING THE AC THE AIR WOULD START TO COME OUT OF THE DEFROST VENT AND THEN COME OUT THE AC VENTS! THE HEAT DOES THE SAME THING AND COMES OUT THE DEFROST VENT! EVERYTHING NOW COMES OUT OF THE DEFROST VENT! PLEASE HELP! THANKS! JOHN

1 answer
when using the max a/c the air will switch at intervals between the defrost, vents and floor vents. never noticed with the heat. its just something older vehicles do.