how to include fan light in dimmer system

There are two ways you can get a close number for the resistor -

1. Use an Ohm meter and figure out the resistance of the bulb you are using.  You can then use a resistor of double that number to reduce current by 50%.  Say you have a 60 ohm bulb at 14.4vdc = 240 mA of current.  If you put a 120 ohm resistor before it you will reduce the available current to 120mA, which will in turn dim the bulb by 50%.  Keep in mind that light produced isn't linear to the eye, so you may need to adjust the value a little bit.

2. Buy a 1000 ohm pot and use that.  You can "dial" in the amount of light you want and then either A. measure the resistance of the pot and buy a resistor close to the measured value or B. tuck the pot away and forget about it.