Just a thought. Disconnect the connector that goes to the blower. Turn the switch on high. If it blows you'll know its not the blower,but something in the harness or dash. If it dosen't blow, install a fuse with say a 1 amp rating and see if it blowes with the blower still disconnected. If it does there probably a short or at least a resitive contact(oil or water) being made somewhere in the harness.
If it doesn't blow with either fuse with the blower disconnected,and the switch on high you'll know its something past the connector that isn't right. I don't know if the blower has a resistor like someone mentioned that is in the circuit when the switch is on high or not. I'm just guessing that the resistor is in the circuit to lower current to the moter when the switch is on a lower setting.
Is there any chance that something is physically keeping the fan from spinning freely(bad bearings, or trash near blades)?