Chuckiman, if your fan is a small one that only takes little current, it should work quite happily off the main 5V supply (5VD as marked on connector CN2), just a little more slowly. I had one running like that when I had one fitted. This will stop running on standby.
Alternatively, you could connect it to the +21V supply (CN8 of the connector) in series with a resistor. I tested it out using one of 100 ohms, and that dropped the voltage down to ~12V. The exact resistor value depends on what current your fan is rated at. Mine is .15A, but in fact only takes 0.1A, so -
Voltage drop needed =9V
R=Voltage to drop/0.1A = 9V/0.1A = 90 ohms. 100 ohms resistor near enough.
If your fan has a significantly different current rating, just replace the current value in the formula above to find the resistor value needed.