This is a very common problem and you need to check for a vacuum leak, broken vacuum line or a loose vacuum line......
Air flow is controlled by plastic doors which are controlled by vacume motors. Could be they're stuck, or a vacume leak.
Most likely a bad or disconnected vacuum hose to the defrost/floor selector in the heater ducting.
the heat blows out the defrost when it should blow out floor vents only
Vacuum leak
2003 crown vic air conditioning vent dont blow
the middle vents only supply air from the ac unit using a separate in dash fan. the floor and defrost vents come only from the heater
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.
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 mode door actuator has failed. We have instructions for repair at www.trailvoy.com
Fuel level, heat or air conditioning up, both high and low, high only, floor only. Center vents are for air conditioning output only, no heat. Heat is designed to blow out side and floor vents only. NOT center. Center is only for air conditioning or summer vents.
the ducts can not be hooked up for the vents to work
When there is a problem with the vent system it dfaults to the defrost setting. Sounds like you have a problem in the doors that direct the air flow.