You have a leak in the vacuum supply to the ac control.
2003 crown vic air conditioning vent dont blow
the defrost vent should just pop out by taking a screw driver underneath the sides of the vent cover and prying it up.
Check under the hood for a broken or disconnected vacuum line.
Your blend door is giving you trouble, it is not switching from defrost to heat or vent
I don't know why the vent would not open for the defrost, but it may be related to the fact that your heater core is blown. If your getting nothing but cold air out of the vents when the heat is on, your core is finished and if the core is finished then it is likely related to your vent not opening/switching. Something''s wrong with your heating either way.
If it's a Laredo, you probably have a vaccum line that has come off or broken. Vent switching is controlled by vaccum in these models. The main vent line comes off of the driver's side of the intake manifold. These are notorious for either slipping off or breaking.
Check for a vacuum leak either under hood or under dash Check the operation of the vacuum controlled motors under dash Check the operation of the selector switch Defrost is the default position for this system
on the dash below the windshield.
The center vent, or defrost duct.
my 1998 gmc jimmy has no floor heat but i have defrost and ven heat
I would think that is a vacuum or a linkage problem with the heater control assembly (where all the buttons/slides are for changing between floor/defrost/vent) Had a diaphragm go bad on one once, and in a jeep cherokee, a slight collision with a guard rail knocked a vac line off for the vent controls, and ac only worked from defroster.
The fastest way to defrost a windshield is to turn the defrost vent on without heat or simply open the windows. Another option is to warm the inside of the car by turning the heater on.