Sounds like your relay has failed. Near the blower motor (follow the wire back from the blower) there is a relay that controls this. It is fairly cheap and easy to replace. Need more help contact me through my board and I will try to help.
you have a bare wire touching on your blower motor
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
switch contact"s
blower resistor is bad
A bad heater blower motor resistor would effect one or more blower speeds other than high.
Probably the blower motor resistor is shot.
Heater Blower Resistor
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