You need to replace the blower motor resistor. I've been there done that.
Heater blower resistor is what controls the low medium high speeds...
The most common cause of the heater blower being on high only is the blower resister failing. The second most common problem is the blower switch going bad.
The blower motor speed resistor is burned out.
the heater resistor needs to be replaced it is mounted in the heater box next to the blower motor
The high speed relay for the blower motor would be found under the hood, on the passenger side of the firewall, close to the blower motor speed resistor.
A bad heater blower motor resistor would effect one or more blower speeds other than high.
Probably the blower motor resistor is shot.
you need a new rheostat !
More likely than not, the Blower Motor Resistor Pack is defective and needs replacing.
try replaceing the switch that controls the low to high for heater It's gonna be the blower motor resistor (near the blower motor).
from what i have read it is the blower motor resistor it goes bad and only lets the fan run on high google blower motor resistor and read on