Horn wiring

The switch in the column supplies a ground that closes the relay, horns blow. That should be a black wire from the horn switch to the relay. One other wire is hot at all times to the relay. Green goes from relay out to horn(s). You have a terminal marked B so we might assume black goes there.
The horn relay moved inside the cab somewhere around 1970 and the wire colors may have changed. Most any earlier wiring diagram available online would show same relay and wire colors. Only difference between yours and a 67 model is the relays location.