The cabin of most cars can get very hot during the summertime. Where I live when the ambient temperature approaches 100ºF/38ºC on a sunny day the inside of my vehicle can go as high as 160ºF/71ºC. This is due to the energy from the sun being allowed to pass through the window glass. Most of that heat is then trapped inside because the lack of airflow doesn't allow it to be dissipated.
Such heat can affect some of the most delicate parts of your camera. If it is left in direct sunlight the long term exposure to UV can also fade the exterior finish and cause some materials to become weak and brittle.
If your vehicle has a separate trunk compartment the temperature in the trunk will stay much closer to the ambient air temperature. It will also stay hidden from prying eyes that might be looking for something of value to steal. On that same 100ºF/38ºC sunny day mentioned above, the temperature in the trunk of my car doesn't get much above 105-110ºF/40-45ºC (as measured by a digital food thermometer that is accurate to within a few tenths of a degree with both ice water and boiling water). My car's white paint probably helps. Several years ago when I had a black car the trunk got hotter, but still nowhere near the cabin temperature which could go up to 180ºF/80ºC!
I've left my gear in my current trunk many times and it seems to have suffered no ill effects from either heat or cold. I've always stored it inside protective bags/backpacks/cases and allowed it to warm or cool to ambient temperatures before removing it from the case.
Keep in mind that most cameras' 'rated' temperature is an operating temperature rating. Storage temperature rating for many devices have a broader range than their operating temperatures. This is commonly seen in the IT world where many commercial grade devices have a wider storage temperature rating (when current is not flowing to a device) than operating temperature rating (when the device is powered). Also keep in mind that an operating temperature rating does not necessarily mean operating the device outside that range will cause ill effects, it just means those are the limits at which the manufacturer has chosen to test and certify their device. Manufacturers tend to be fairly conservative in this regard.
Consumer goods, including cameras, are routinely transported via shipping containers that can reach internal temperatures of 120-130ºF/50-55ºC near the top of the container during the day for weeks at a time when transported in direct sunlight on the decks of ocean going commercial ships. The same is true for large transport trucks and rail, although high value products such as consumer electronics rarely stay loaded on a container or truck more than a few days once it has crossed the ocean.
The current Canon flagship, the EOS 1D X Mark II has an operating temperature rating of 0 – 45 °C, 85% or less humidity (32 - 113ºF). Canon doesn't publicly release storage temperature ratings. Nikon recommends not storing cameras below -10ºC or above 50ºC (they rate most of their cameras to operate at 0-40ºC). At major sporting venues with artificial turf the surface temperature can reach 130ºF/55ºC on sunny days. Ditto with closely trimmed natural grass on top of a layer of sand to enhance drainage. That's the main reason you see a lot of shooters required to kneel wearing knee pads at such events. They lay their Canon and Nikon cameras and lenses on that turf routinely.
Here's a link to a copy of an operating manual published in 1969 for a large airborne reconnaissance film camera used by the U.S. Army. The section regarding Arctic conditions states the camera may be operated as cold as 0ºF/-17ºC and stored down to -65ºF/-54ºC. It also warns against moisture and condensation as being more problematic than temperature extremes.
Notice that in this blog entry featuring tips on how to deal with hot weather from Chuck Westfall, Chief Technical Advisor at Canon USA for many years, he is much more concerned about humidity and condensation as a byproduct of moving from a cool to a very hot, humid environment than he is with heat itself.
Just be sure the camera has had a chance to cool down a bit before you start using it if it has been in an extremely hot or cold environment. Also be sure to protect it from condensation during the transition period by keeping it in a bag or protective case.
