Most likely a bad or disconnected vacuum hose to the defrost/floor selector in the heater ducting.
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.
The doors are electric. You most likely have a broken door inside the heater box under the dash.
The heater may not be on - but the fan still blows air through the vents when they're open !
There may be several reasons why the air only blows out the front vents and not on the floor or the defrost on the 2003 Dodge Ram. The fuses may be blown, or the heating coil is not working correctly.
A vacuum leak, or one of the doors is sticking/bad vacuum motor in the heater box.
That's probably steam from a leaky heater core.
the heat blows out the defrost when it should blow out floor vents only
it could be the heater is leaking!
heater on at idle and will not blow warm air. when moving it blows the warm air from the vents. what will cause this
Your average A/C unit blows heat through all vents.
It's either the thermostat or the heater controls.