The steering cable's core wire is broken. How can this be repaired?

Steering cables cannot be repaired. 

If a steering cable inner core wire separates or breaks within the steering helm (gearbox), the steering cable must be replaced with the correct one for the helm. 

The helm may need to be replaced as well. Cable breakage within the helm may have caused damage to the gears. A helm cannot be repaired and must be replaced as a unit.

If the correct replacement cable and/or helm is no longer available, replace the helm, bezel, and cable with an appropriate Dometic steering kit.