Comment by ErroneousBosh

Yes, the whole thing is built like bike brake cables, with a spirally metal support core, the goodies up the inside, and then a nylon braid and PVC jacket over the top.

I have issues with the PVC-jacketed cables under the bonnet of my nearly-30-year-old Landrover, where the plasticiser has been baked out of the insulation and they've gone brittle. Worst affected are the wires to the fuel injectors and the lambda sensors, presumably because the former are at the top of the engine and get reflected heat off the bonnet, and the latter because they're near the literally red-hot exhaust downpipes.

That's okay for an old vehicle that you'd expect to repair, though.

I've seen the same problem in three-year-old Toyotas, and that is Just Not On.