Horn does not work when truck is moving

Hello, 2003 has the shifter on the column, correct? This is likely a case of a short in the steering column, perhaps a wire being pinched when the shifter is moved. Have you already removed the steering column cover to inspect? Is it blowing fuses? If you power the horn at the relay while the horn doesn't work at the steering wheel? This will at least narrow it down to one side of the relay. Keep me posted, and I'll help as much as I can.