The other good reason to go the switched relay path is that if you decide to run a temp sensor to switch the fan on and off then it will be easy to wire in a "normally open" temp switch to the earth circuit and run the + signal wire straight to battery voltage.
This will then mean that the fan only kicks in when the temp sensor/switch reaches it's preset temp and earth. You can choose different temperature sensor/switches to match what temp you want to stay at.