I guess there's enough interest to continue with the thought...

After looking at it some more, I don't think I'll be able to do it the really cheap way I may have to throw a microcontroller at it

It would cost a little more, but it would have much better control over turn on and full speed temperatures, and would open the door to switching the current of the fan itself (for an HHR type fan without a trigger wire , and no wimpy 30A limit either ).

I think it would work best if we could install another temp sender that I can test and that I know the graph of voltage/resistance is, like this cheap one.

-OR-

I make a little box that puts out an adjustable duty cycle (up to ~90% on) whenever it's turned on. ie, you can change the speed of the fan, but it'd be running at that speed all the time. This would be the cheapest solution.

(Should we keep this in this thread, or start another one?)