2006 5.4l. AC blows weakly cool when coming through dash vent. Barely cools the truck. Thought it was low, added a can, still nothing. Now. Driving down the freeway today, I hit the button that splits the air between the dash vents and the floor vents, and it nearly instantly got colder. Is that even possible? Also, can someone make sure I know how to add a can? AC on high, not the nozzle the closest to the grill right, the one further back? This is driving me nuts! Ideas?