could possibly be the blower motor resistor. which is generally located on the passenger side under the dashboard by the blower motor.
Odds are you need a fan switch, It is very common for the high on GM fan switches to go out.
The fan control module in the engine compartment has gone bad.
try replacing the blower motor resistor
Can't get my heater to work on my 156
Defective Heater Resistor Pack.
Check the low speed relay and the resistor at the motor
You most like have a bad blower motor resister.......
A broken fan heater can not work and A FORD Foucs does not run
The blower motor resistor is bad and goes to high fan.
try replaceing the switch that controls the low to high for heater It's gonna be the blower motor resistor (near the blower motor).
2004 Chevy express van, heater has no high speed, at one time the blower stayed on even when the ignoton was turned off therefor draining the battery,
Is there a battery in your vehicle. If not put one in as the fan is electric. A good indicator is if the dome light comes on when you open the door. If it does then there is a charged battery in your vehicle. Maybe the fuse is BLOWN.