Yes, the coding will also need to be done. I don't think it'll be too complex just to get it working. After its working I can think about improving it. The first step will be open before e-fan turns on, close around the thermostat temp.
I am planning on tapping directly into the coolant sensor signal on the car. It shouldn't be hard to find, just one of the many wires behind the glove box I'd assume. However, I'm not exactly sure how to proceed as far as reading the coolant temp sensor signal with the Arduino. From what I've read its just a thermistor. So, to get a voltage reading from it I believe I'll have to create a voltage divider (unless you guys know of a better way, I am no electronics guru) so I can get a voltage reading from it.
I do have this resistance chart for the temp sensor. It looks like the range we're looking at is between 250 ohms and 300 ohms. That seems pretty tight, but again I'm not electronics wiz.
I'm also contemplating how to control the open and close set points. Part of me thinks a 20 turn pot would work real nice for adjusting things. The other part says throw a laptop in the car and adjust the code.