Makes sense on the manual override. That's what I'd want too. On / auto / off

I also would be concerned with battery drain after the car is off. Here's an idea for that ...

Most circuits in the car are switched off after 16 minutes when the car goes to sleep (I think key out and doors all closed triggers that). So if you found one of those circuits and ran a wire from the fuse box through the firewall into the engine bay, and used it to turn a relay on, to unswitched engine bay power, that would give you an extra safety against running down the battery. Of course it would make sense to design this and your on/auto/off control together.

If for some reason the controller or sensor malfunctioned or you parked on top of a volcano, even if the fan would want to stay on, the sleep mode would turn it off after 16 minutes.

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2001 X5 3.0i, 203k miles, AT, owned since 2014