I don't think you are doing anything wrong. There is no polarity to a resistor. Each end is the same. If you have hooked up the resistor in series with the heater motor(as it should be), you are splitting the 12 volts between the motor and the resistor with the same current running through both. 6 volts gets dropped across the resistor and 6 volts gets dropped across the motor. Each will dissipate the same amount of power. That's why the resistor is getting so hot. About the only mistake you could make is to hook the resistor up in parallel. This would mean that one side of the resistor would be hooked to 12 volts, the other side to ground. Now you would be dropping the full 12 volts across the resistor which would really make it get hot, and your heater motor would also still be getting 12 volts.
