A local auto

UV rays are a lot of the problem. More than heat.  Eats many
plastics, etc for lunch. Also often fades and discolors plastic
trim pieces. The best way to protect is to use the window shades
when you are parked in the sun.  I have them for front and back
in the accord.  I hate cracked dash panels and padding.  It's the
UV rays that make the stuff get real brittle and crack.  If you use
the shades, the dash will look like new nearly forever.  Course,
they are kind of a pain sometimes, but I think it's worth the hassle.
I use the kind that you twist into a circle frisbee size shape when
not using.  The shades also keep the sun off the steering wheel.
The black honda steering wheel on mine gets real hot if sun is
shining on it through the windshield.  That can lead to cracking on
the wheel cover.  I cover the rear window cuz it's carpet/plastic
covering is prone to UV damage, and it also cooks the rear speakers
after a while.. Just covering the front/back windows can reduce the
heat a goodly number of degrees if the car is in the direct sun.
I'd like to rig up some side shades of some type.. I often have to
park that car out on the street, and the sun really nails it in the
afternoon.
MK