How I set my extruder fan power was to tap directly into the 12V power feed on the control board and to the extruder heater output. The power tap and switched leg of the extruder power feed a small timer/switch circuit board. The board has a 5 minute delay timer so when it first sees power to the extruder it starts a 5 minute power on timer that keeps the fan on for 5 minutes after the extruder has been turned off. This prevents the cold part of the extruder from heating up and saves wear and tear on the fan.