That could be a short somwhere or any of the three components involved shorted.
a. The blower motor
b. The Heater blower series resistors
c. Your blower speed switch
To discard the Heater blower resistors, test it on max speed (number 4 I believe). If that still blows your fuse, then it is between the motor and the switch.
To discard the motor, connect the motor ground directly to any ground on the car and test. You can see some diagrams and connector info on these links:
Link 1
Link 2
Link 3
If that still blows your fuse, then could be the switch or a shorted wire in between.
Hope that helps.
