If you want the horn to remain "hot" even with key off, there's the option of
adding a battery cut-off switch in trunk next to the battery. This way there's always
the ability for full battery isolation where all circuits are cold. It's also a nice safeguard
against any known/unknown parasitic draw that can kill a battery during storage. It
also allows for less reliance on a battery tender.
Just remember that the cut-off needs to handle peak current demand so get a good one.
Flaming River's "Big Switch" is $125. Pricey but hey, buy once cry once.
Here's a link to said good one:
https://www.flamingriver.com/index.p...0006/FR1044LED