How to repair cable insulation

Hi @japaw

Since the wiring doesn't appear to be damaged only the insulation, try using some suitably sized i.e. same diameter or just slightly bigger, grey coloured heatshrink tubing to cover and protect it. (examples only)

Cut two pieces long enough to cover the damaged area and then slice them open lengthwise.

Place one length around the damaged area then the other length on the other side so that it covers the open length of the 1st piece of tubing and then apply heat, back and forth, evenly along the tubing (barrel of soldering iron or heat gun), to shrink it onto the cable so that it remains tightly in place.

Normally heatshrink is slid over the end of a cut cable first, then the cable wiring is joined to another cable etc and the heatshrink is slid back to cover the join and heated to seal it, but since the cable is not cut you can try the above.