I think the PVC insulated wire's insulation will eventually fail because it can't withstand near the temperatures of a boxed in engine compartment of one of these old trucks. I can't prove this, however, so I just use common sense: PVC is only good to 185 degrees Fahrenheit and engine compartments can get over 300 degrees. I think it's just a matter of how fast this will occur in a truck that isn't driven that much. It might take decades or just a few years for the PVC insulation to deteriorate to a point where problems occur in a truck that is hardly ever driven, I don't know. That's why I posed the question.
If I was building a wiring harness for something that mattered that was driven daily I wouldn't skimp on the wire. Volvo skimped on the wire on the 240's they built and they ended up with a bunch of cars that had to have their wiring harnesses replaced or repaired. Anyone that's ever owned one of these cars (like me) will tell you old 240's had crappy wires.
Looks like PEX wire is only used in the construction industry for building and overhead wires:
http://en.wikipedia.org/wiki/Cross-linked_polyethylene#Other_uses_for_PEX . I can't find any such thing as PEX wire used for vehicle wiring, just the cross-linked wire types I mentioned earlier. Google it up.
