Plastic parts tend to shrink with age as they loose volatile chemical components. In a hot car, this shrinkage can happen much faster:
This Martin was repeatedly left in the car at festivals. It never got hot enough to melt glue, but it did get hot enough to accelerate the aging process. Here the pickguard shrank and curled, taking the top with it. Notice the two cracks that are starting to shred the top at the edge of the pickguard.
In addition, the entire instrument was covered with a gummy haze all over the finish. This haze is a result of repeated overheating. In the heat the moisture is driven out of the case lining and interior of the guitar, condensing on the finish as it cools. The gummy residue causes no problem and can be wiped or buffed off, but it is a clear symptom of overheating.
Here's a fingerboard that shrank in the heat. The moisture was driven out and the wood shrank enough that the fret ends stick out like little spears:
These guys are sharp enough to cut your hand. Not just annoying, they too are a symptom of overheating. Once the wood shrinks, no amount of rehumidifying will cause it to regain its full original size. The sharp fret ends are easily repaired by filing level with the fingerboard.
What's not easily repaired is the other damage caused when the fingerboard shrank:
This is the same classical guitar fingerboard. It was originally flat, but is now cupped and concave through its entire length. Unfortunately, because of the heat the frets are also slightly loose as well. The only way to make this guitar play really well is to take out all the frets, level the fingerboard and install new frets. A big job that would not have been necessary otherwise.
Please watch out for parked cars. Your car won't get hot while you're driving, so when you stop, just think of you instrument as being a living thing inside the car. If the heat would kill your dog, it could also kill your guitar. The same thing goes for other sources of high heat: direct sunlight, heaters, fires, etc.
Now, if you're travelling, you may have your sleeping bag with you. You can protect your instruments in the car trunk by covering them with LOTS of insulation. After all, if you sleeping bag will keep you warm, it will also keep your guitar cool. Insulated and reflective case covers help, too. Only, don't count on them for long term protection. Better to avoid heat altogether.