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This horn was ordered for testing, as it was both UNECE Reg 28, low (ish) power consumption (4.5A vs the 9A of the old horn): https://www.amazon.ca/gp/product/B000DINKPQ/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

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Josh Rong have discussed the horn implementation, as the horn is turned on when connected to a supplemental pack.

The horn button will be on steering wheel, and when pressed will pull the GPIO low on the steering wheel interface board, giving us indication that the horn is being pressed on the CAN bus. The CAN bus will then tell rear power to enable the load switch to turn on the horn. Rear power will be providing the horn with power from the same supplemental battery used for driver ventilation.