On my 59 100-6, there are 4 wires going up the steering shaft and to the trafficator & horn.
The GREEN wire carries 12 VDC power and switches it to either
the GREEN/YELLOW for the LEFT turn signal OR to
the GREEN/BLUE wire for the RIGHT turn signal.
The BROWN with BLACK STRIPE wire carries 12VDC from the horns and grounding this wire should sound the horns.
Check to see if you have 12VDC on the BROWN with BLACK STRIPE wire.
If you do, there is a bad ground within the trafficator unit.
If you do NOT, the lack of voltage is due to an open along the wire(s) or at BOTH of the horns or at the fuse.
Tim