I would just double check that circuit I think there has to be something else on that 40 amp relay is way more than a horn would pull ,unless it was used a as air raid alarm in England years ago.
The closest I've been to needing the horn was a few weeks back. We had over two feet of snow overnight and there were three of us neighbors on tractors running around plowing driveways. Here's what the snow did to my greenhouse:
Sounds like a good plan to me. Cleaner then putting in a push button like mine got for the glow plug upgrade. I say if the circuit can handle it then go for it.
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Here's what the snow did to my greenhouse:
