As it happens I've just finished my fan controller. I mentioned I was going to have a go at designing something after initially fitting my fan some time ago. I designed it and bought all the components several months ago but I've only just got around to actually soldering the components into a bit of Veroboard. My knowledge of electronics is very limited but it seems to work. It's sitting here on my desk at 32 deg C where it would normally be at about 43 deg C and I can't hear the fan unless I get my ear really close to it. I'll have to see how it goes in the car (where it would always top 55 deg C after about 1 1/2 hours driving.

The fan does increase in speed as the Thermistor gets hotter (tested by bringing soldering iron near to Thermistor) but I've yet to correlate temp against output voltage. I've attached a scan of my original circuit diagram and an Excel spreadsheet of all my calculations in case anyone's interested or wants to take my design and improve it. As you can see from the circuit diagram part of it didn't work so I removed it. That bit was supposed to keep the fan off until it reached about 6V.

I'll do some more experimentation & take a few measurements when I get time. Meanwhile someone with more electronics knowledge might like to improve it.

You may notice I've used a Wheatstone bridge and an Op-Amp (LM324 - quad) as mentioned in a couple of recent posts in this thread.