I use to have a Ready Brake system when I was towing with a Motor Home. By looking at the cable with 3 broken strands you might have had the link cable abit to tight which is why the cable was damaged. Plus, it was a good thing you had the indicator light installed to tell you when the unit was engaged.
BUT, wait just one minute, if you where driving down the road, or stopped why was the light on in the first place. The Ready Brake unit only activates when your actually stopping. And once your stopped the unit goes back to it's normal state. Just something to think about. Now, if the only problem is with the broken cable I'd replace it with a new one, and this can be done by going to any hardware store and buy the right size and length of cable, and with a couple of cable clamps and your back in business. Fact is, I've still got the Ready Brake unit.