The air only blows out only on defrost on my 91 chevy truck

Which vents, floor--defrost--dash? Can you hear the blower motor running? The squirrel cage could have broken loose from blower motor? There could be an actuator problem under the dash? If actuators are vacuum controlled and vacuum is lost, it might default to defrost, only blow out defrost vents.

Possible blend air door malfunction.

i believe this is a vacuum operated system. if there is a problem with the vacuum supply to the control unit, it will automatically default to defrost. i would start by checking the vacuum source lines. good luck!

try to check the fluid level or change thermostat

the heater & defroster baffles are controlled by electic motors that open and close the baffle (doors) to let the air to the heater and defroster vents. the motor must be bad

check defrost actuator on air box..located just above the gas pedal to the left..posibly broken or spring has slipped out of place

Typically, you have a vacuum issue, air flow system is designed to FAIL in the defrost mode, so no vacuum, doors are spring loaded to blow out defrost ports.

Blowing out of the defrost is a vacuum problem. You have a leak between the motor and the heater controls. Most likely needs to have the A/C system charged up.

most american cars force the defroster to blow cold air, this is not a problem, it does this by design.