What happens when you leave your GPS in the car under a blazing sun and its batteries are not in the best of conditions? Well, it may explode and burn -- like this one -- melting your dashboard.
And since you don't know if the batteries are OK, the best thing you can do is never ever leave it in the car to overheat. Unless you like the just-hit-by-a-TIE-Fighter Millennium Falcon look. [Walyou]