I have a 99 f150 5.4l and a few months ago when it was a bit hotter out I would go to put the ac or ac max on and heat would come out no matter what setting i put it on now about a week ago i started my truck and went to put the heat on and the ac works now but no heat. yesterday and the day before i figured i would try it and it worked but when i turned the blower off then on again it stopped, any idea what it could be and how i would go about fixing it.. when the blower is on panel, floor def or floor its just pulling outside air no temp change. when i put it on ac, ac max ice cold air blows, i have already changed the temp blender on the dash no difference.