You will need a multi-position switch also of course. Anything done like this I like to give a week or more for the silicon to cure before subjecting it to the elements. Conversely you could mount the resistors on an aluminum plate using silicon caulking, then add the wiring and silicon all the exposed leads and connections too. I would put them in an array that mounts in the air stream so that the parts don't get more than warm. If you wanted a slower rpm than this would give, then make the resistors 1.5 ohms each.

Other speeds would be by bypassing one or more of the series resistors.

You would have full power by applying 12v directly, and you would have lowest power by using the full length of the resistor pack in series. That would then give you an easy way to have a speed range. Three 10 watt, 1 ohm resistors could be wired in series to give you the dissipation ability without the parts getting too warm. The resistor value would then become about 3 ohms. The current draw will drop somewhat because of the reduced air flow at lower rpm, but essentially you will be drawing about 2 amps or so. If you use a resistor to drop the rpm, you would be dissipating about 15 watts or so if you set it up to run on about half voltage. Simplest way to get variable speed, technically, a big waste of power, but in your application, who cares. Or rather than a switch and resistors, get a 3, or better yet, a 5 watt rheostat. P=IE Power, in watts, equals current times voltage. Next calculate how big a resistor is needed. R=E/I so the value of the resistor (in ohms) would be 9 divided by 0.250 (milliamperes converted to amperes). Say the fan draws 250mA and you want (about) 3/4 speed. Find the current draw of the fan and calculate the value of the resistors you need using ohms law E=IR, where E is the voltage desired, I is the current requirement of the fan in amperes, and R is the value of the resistor. For a variety of fixed different speeds, a rotary switch and a few resistors will do the job. For variable speed, Amazon sells a 120vac input, variable voltage DC output for about $10. Do you want 2 or three fixed speeds or variable speed.

If, so, you just need to change the voltage applied to the fan. I assume you are talking about a 12v DC computer type fan. Think of it as the control for a three way light bulb. The ceiling fan switch is designed simply to connect different sets of wires within the ceiling fan.

12 volt dc fancontrol