Power source suggestions? I'm all ears

[AdamIsAdam]

Howdy all. Last night I read an old thread about the Stebel Nautilus air horns and by the grace of Amazon, it arrives tomorrow. And with COVID on the brain (ie, I'm bored), last night I started wiring.

Before I ask this question, you should know that I didn't build the car, so I'm not sure exactly what wires were used and not, but the builder did do a phenomenal job notating right on the wiring diagrams, so i'm pretty confident.

Question: I'm thinking I'll put the horn relay at the horn to minimize wiring.
(this vid shows the horn I ordered and wiring necessary https://www.youtube.com/watch?v=tK9lB1hH-8w )

So since the horn DK Green and Black wires get plugged into the relay, if I run a 14ga wire off a power source capable of delivering 18 amps, this seems to be the simplest way to wire the thing up with the least amount of wiring. So last night I ran one power wire from the horn, inside the factory harness, all the way inside the car.

GET TO THE QUESTION ALREADY!

I'm debating tapping into the unused heater BRN-HEATER wire coming off of the 20amp fuse in the panel which is switched power, or tapping the heavy battery wire coming up on the passenger side from the battery, which is constant of course. I will put an in-line 20amp fuse. (I think there are two red power wires coming in on the passenger side. I think one feeds the fuse block and looks like 8ga or something very thick, and one is thinner which I assume feeds toe ignition switch. Am I OK tapping the thinner one?)

Is it better to have constant power to the horn at the risk of having an always hot power wire, or is it better to have switched power to the horn and know that wire is dead when the key is off.
(I located that heater wire above drivers right knee, verified it's the right wire by pulling the fuse, and I don't THINK it's used for anything else.)

I'm inclined to go with the constant power, ALSO ASSUMING that tapping into the main feed wire is not a problem.

Thoughts? Other suggestions on pulling power from? I'm all ears.