Have 4 wires off the harness at base of firewall below brake master. 3 wires go to rear tail lights and lic plate light. 4th wire has unswitched power to it at all times. Horn button??

 

What color is the wire?

Is it red? Thinking possibly for the brake light switch?

 

its a yellow wire with a green female bullet end. Brake and running lights work. Turn signals not working but don't see anything for this to connect to in wiring harness

 

I just checked my '57 and I have 4 wires from the firewall. Three go to the back of the truck. The 4th routes to the steering box, which would make it the horn wire. I'll take a pic if you need when I get home from work.