If someone wants to break into your car, they'll see something inside and then attempt to break in. That, or if they know anything, theyll just smash your window, check your trunk through the seat, and leave.

Leaving your window cracked probably doesnt increase your likelihood of getting your car broken into, especially if you dont leave anything valuable in the car.

Either way, leaving things like backpacks, large cases, really anything at all valuable looking anywhere in your car is the greatest way to get your car broken into.