Electrical Wiring Assistance

[Stang70Fastback]

I don't think it should matter, as I'm splicing into the wires between the relay and the horns themselves. There way be multiple circuits that trigger the relay, but there is only one relay and wire that runs to the horns themselves.

[ls1ac]

I see three possible problems with your diagram
first the horn switch is to ground, you have it on the hot side.
second the added switch is momentary meaning you will have to hold down both the horn button and the switch to get the horn to work.
and third and most important the second relay has no ground. so the new switch will do nothing. I also believe you were trying to use the cig lighter to power the new horn so you will not burn out the fuse for the oem horn.
Use a single pole double through switch to change the output of the horn relay from oem horn and the new horn relay. Use a single pole relay to supply power from the cig lighter or directly through a fuse from the battery to the new horn.

[ls1ac]

I see three possible issues with your wiring diagram.
First and not critical the horn button in the ca is to ground not hot.
Second your added switch connects the alternate sided of the added relay and there is no ground so the relay will do nothing when the switch is pressed.
The next thing is that the switch you added is momentary and even if wired correctly would require pushing both the horn relay and your switch.
If you install a single pole double through switch on the output of the of the stock horn relay you could select either the oem horn or the control terminal of the new single pole relay. I believe you were trying to use an alternate power supply so you do not burn out the horn fuse, power from the cig lighter or better yet directly from the battery through a fuse to the new relay.

[Stang70Fastback]

Alright, so I've taken your feedback and I think I've fixed things.

1. I moved the switch to the ground side. This is just for reference, since this is meant to represent how the car comes from the factory, but best to have it as accurate as possible.

2. I swapped my momentary SPST switch for a momentary DPDT switch. This allows me to wire it to the relay so that I can send +12V of current one way or the other, which will energize the coil in one direction or the other, thereby flipping the relay one direction or the other. I want to use a latching relay that remains in whatever position it was last energized in (in other words, a spring-loaded relay that only requires power to change position.

Does this seem closer to what I want? I realize the wiring is a bit confusing at first, but I think it all makes sense now.

[Ultramaroon]

It would work but is rube goldbergesque. What you show there is not a momentary switch either.

Just put in a regular old spst toggle switch to energize the horn select relay when you want to use the air horn.