Two cents worth on softening dried or stiff wire/cable insulation:
First of all, a "one size fits all" approach will not work because different cables use different materials for insulation. Having said that, here are a few observations I've made over the years, along with a few tips that may help to soften cables:
Wires can stiffen for reasons including and not limited to any combination of the following reasons:
1) Certain types of plastics when brought into contact with certain types of oils, can result in the insulation stiffening in varying degrees that in some cases will leave the plastic so brittle that it will become extremely hard and brittle, and will crack if the cable is flexed in any degree.
2) Heat and temperature changes (whether cumulative effects of many years of seasonal changes, or from having been exposed for shorter periods of time to heated conditions.
3) Ultraviolet light exposure (can produce changes in color characteristics as well as changes in pliability).
4) Gradual dehydration or drying out of the insulation over time.
I haven't tried to find solutions to the brittle cable problem, but I have found that the following treatments will work on various cable insulation materials:
1) Lucas Power Steering Stop Leak. (this softens rubbers and plastics, and will probably work on PVC and Teflon in some degree). This is the thick gooey dark-red gel.
2) Whitestone Renewal Gel. (softens rubber, plastics and other materials - not as drastically as the power-steering stop leak, but it does soften the materials over time.
3) WD-40 (this works more on the surface of plastics, although it may work it's way deeper into the plastic if allowed to soak over some time.