Firstly, hire a car with an enclosed boot (trunk), and lock all your valuables in the boot.
Second, do not leave any food items in the car, not even in the boot, or the bears will do whatever is necessary to break into the car; smashing windows, etc. Bears are fully capable of doing precisely that. Even something as small as a piece of candy inside a car can be smelled by a bear.
Bears have been known to smash the wind-screen or widows, crawl in, and then rip up the back seat to get at food stored in the boot of a car.
