85 needs to be connected to 12v, not ground.

I think you may have just solved my problem. When I’ve used relays in the past, pin 85 was connected to ground, since I wanted the relay to close when the trigger went high. I’m not sure why it never occurred to me that I’m essentially trying to do the opposite thing here, since the horn is triggered when the trigger wire is connected to ground.

I’ve never worked with individual diodes, so I’m not sure about the correct terminology, but which way would I want the diode to “face”? Do I want it to allow current to run from the 12V source, through the added relay, to the horn switch wire, or the other way around?

Additionally, would I need to add an in-line resistor? It makes me a little nervous connecting the horn switch to 12V, given that I doubt it’s designed to carry a significant amount of current.