probably the heater core is plugged
check the blower motor resistor
Under the hood in the fuse block you will see a large black block. This is the heater blower fuse.
have you checked the fuse if so then you will need to replace the resistor for the blower hig and low dettings have you checked the fuse do you have 12 volts to blower if so then you will need to replace the resistor for the blower replace the blower motor
Defective Blower Motor Resistor Pack.
There is a large amount of leaves or pine needles in the blower wheel. It most likely gets worse the higher the blower setting, the faster it spins...........
Remove the wiring harness from the back of your 2005 Chevy Cavalier heater blower motor. Remove the heater blower motor retaining bolts. Remove the heater blower fan.
Defective heater blower motor resistor pack.
The blower "switch" is the knob or lever that you turn or move to adjust the fan speed. Located right on the climate control, next to the temperature setting.
Heater blower resistor is what controls the low medium high speeds...
I changed the one in my car it's located behind the glove box just remove the glove box take out the screw that holds the blower sensor in place unplug it and install the new one.
Normally when a heater blower motor only works on the High setting, it is caused by a defective Blower Motor Resistor Pack. The fix is to, Replace it. In very rare instances it can also be a defective blower motor speed control switch, but rarely.
The blower may simply have burned out. If the blower is out then the heater probably still works.