I had the same wiring question with my 49 GMC. No relay on the wiring diagram, no wires in the harness for a relay, nothing on the firewall.
On the other hand the diagram and wires were for a 6-volt system, which mightn't have done any harm wired directly to the horn. But, since the horn wire in the steering column is always on, it seemed like a 12-volt system would burn it out. A relay, using lower voltage, seems like the way to preserve it.
