Hot wire

Does anyone know which wire under the dash is the hot wire going to the factory horns.

Right now I have picked up the power from one of the factory horns under the hood but I would like to pick up the hot wire before it splits to the horns. The factory horns sound a split second before the train horns blow but I don’t like to give any one that warning.

I had the same horns on my F250 and picked up the hot wire from the back of the fuse box and the could toggle the factory horns and the train horns through a relay and a switch on the dash.