I’ll assume you’re trying to hatch some kind of poultry or game birds in your incubator. Your fan setup needs to be tailored to the kind of eggs you are hatching. Some incubators use the fan for venting – to prevent overheating or condensation of moisture. Most though, just use the fan to circulate air to keep the temperature even throughout the box. Most eggs will need a source of moisture in the incubator – and the high humidity can cause problems with some open-frame thermostats. Ducks and other waterfowl need an especially humid environment, so a sealed thermostat is a good idea, if that’s in your plans.
For starters – you might consider using a 24 volt fan, or a very small 12 volt one – to keep airflow as gentle as possible. For a 25W 12V lamp – I would use a low-voltage RV (caravan) incandescent bulb. Ebay has them for sale right now from $1.35 to around $3.00 each.
Wiring is as Steve described it – Starting from the positive (red [+]) side of the input, wire through a fuse; to one side of the thermostat; out the other side of the thermostat; to the positive (+) side the fan, which should also be connected to one side of the lamp socket. The negative (-) side of the fan should be connected with the other side of the lamp socket, along with a wire connected to your negative (black [-]) input.
Make sure your incubator is ventilated; keep the eggs out of the water pan; mark one side of each egg with a pencil “x”, to help keep track when you periodically turn the eggs (critical). Keep a spare bulb handy, and an eye on the bulb in the incubator to make sure it hasn’t burned out. Good luck!