I melt plastic for a living---primarily extrusion. I've messed with
the pieces left over after a G17 grip frame reduction, and they
appear to be a nylon of some sort.
Typical castable nylon properties:
Continuous use temperature 200°F +
Softening temperature 300°F +
Melt temperature 400°F +
So----Nope, the heat in a truck won't hurt it. If it is in direct
sunlight it would probably affect it after long exposure, most polymers
eventually show some signs of UV degradation.