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The 1979 Chevrolet Corvette air flow door might the stock in the position so that air only blows out of the floor vents. The air flow door is located next to the blower fan.
The blower might need replacing. Normal cause is a failed blower resistor, caused by a blower fan that is failing.The resistor and fan are located above the passenger side floor behind the glove box.
blower motor resistor
your switch cable might be broke you will have to remove your dash
The blower resistor power module has failed. It is behind the glove box.
Check your servos.
Your heater core is leaking. This is the part of your car that creates heat for the passenger compartment of your car. There is no fix for a leaking heater core except to replace it.
behind the blower motor. to take off the blower motor its on the passenger side under the glove box but above the floor. unclamp the casing where the floor light is. then theres the blower just unsrew the three srews. unhook the hose, unclamp it then then take it out
the heat blows out the defrost when it should blow out floor vents only
Something like this just happened when my a/c blower was just changed. The mechanic forgot to plug in some of the wires around my blower which caused the air to blow out the dash and the floor only. It was three wires (one yellow, one white, and one red)that my husband pluged in. After that it worked fine! Just a suggestion!!!
Sounds like the heater core is leaking its a big job probably best left to the professionals
Check upstairs heating unit.