Just a quick update.
I replaced the resistor with a trimmer. At about 600 ohm it seems to quiet the fan done quiet a bit. Where it used to run 31°C on my desk, it now runs 33°C, so I might even lower it more.
I haven't yet had the chance to test it in the car (this morning it only got 41°C with the fan off). I'm hoping it'll be hotter on the way back home.


Frank