1.) You have the relay coil on always on power but not the power for the relay/horn its on switched power.
2.) You could tie 86 and 87 together and run them to always on power distribution block with short wire as much as you can. then run a grounding wire/switch on pin 85.
3.) This way, in case of severe accident, you only have to worry about that one single wire not breaking to keep the horn running not 3 of them(the one running form the relay to the horn, so protect it well and have it a bit longer for stretch).. The switch wires are used as ground/signal in this configuration, so if it short to chassis it complete the loop and just keep the horn running(better that no horn at all). the big power wire usually dont break as easily as small wires. So we try to use the small wires as ground/signal for this kind of stuff(it also benefit for corrosion protection too, ever notice how its always the positive wire that corrode first?
).
and It also keep the small wire short, since you can just ground the other side of the horn switch. Cheaper and safer.
Yeah i know its probably overkill, but just saying.