The 1984 Buick uses vacuum lines to change air flow from defroster to ac vents. Check the entire vacuum system for leaks.
The vacuum supply line from the engine is disconnected or broken. When the vacuum supply is cut off the system defaults to the defrost mode.
Vacuum leak
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.
If central air conditioner turns on but is not blowing through the vents, check the filter. If it is dirty, change it. Change the thermostat into cool, this will defrost the air conditioner.
Most likely a bad or disconnected vacuum hose to the defrost/floor selector in the heater ducting.
If the heat selector will not switch from defrost to the dash or foot vents on a 1992 Buick, it is possible that the cable connected to the vent is broken. When the selector is moved from defrost, a cable opens the other selected vents on the heater.
You have a vacuum leak causing your vents to switch from dash to defrost.
the heat blows out the defrost when it should blow out floor vents only
When there is a problem in the vent system it defaults to defrost setting. Check the doors & vac supply
You probably have a jammed or otherwise defective vent door.
Could be a vacuum problem--either broken or disconnected hose or bad vacuum motor under dash or coming in through firewall Defrost is the default position for this system
You lost the vacuume supply coming from the engine, defrost is the default mode if a mafunction in the vacuume supply is lost.