Tom wrote:
I wonder if that would fit under the hood of my pickup?

It would be difficult, but not impossible on a full-size pickup.
Overall length is 23", horn mouth diameter is 9". It's possible that
you could find a spot where the back end of the horn was near the
engine, and the mouth was firing straight down at the ground, level
with the bottom of the engine compartment.

There are other places under the truck that this will fit more easily,
but with increased risk of theft and possibly increased visibility.

There are other considerations also, this type of horn is arguably too
loud for road use at full pressure, and it's borderline too loud at
minimum sounding pressure also (around 40 psi). Not for use on
pedestrians! :-)

-Adam