The original wiring harness on my 5700 Chevy cabover has the two wires for a horn, taking off the harness in a similar location as a 1/2 ton (near the origin of the left headlight wires). The previous owners clipped these wires and attached wires to a double horn mounted on the passenger side firewall. I would doubt the original horn would be mounted on the manifold like a half ton (you would be sitting almost on top of it in a COE!!).
Does anyone know where the original stock horn was mounted and how long were the wire leads from the harness itself??