Thanks for ideas, all.

I'm thinking the "behind the louvers" placement might work well as long as
the sensor is out of direct sunlight. Anywhere near the headlight I think
would falsely increase the temperature, esp. at night...The wheel wells and
under the van in general I feel is too exposed to water, and if you really
waterproof the sensor, then how about changing its little battery when the
time comes?

Strangely challenging problem, inconsequential though it seems. But as the
globe warms, I wanna know!

Best to all.