People will try to tell you to perform all sorts of rituals to keep the glass-breakers away but it won't do anything. These guys have a rock that keeps tigers away. "I have always placed a sign on my car window that says 'nothing in car' and no one broke into my car". Yeah, homie, but it wasn't that sign that did it because I've seen one of those signs hanging by tape with the glass shattered. I've seen shitty cars full of trash with their glass broken. I've seen cars that haven't a damn thing visible with their glass broken. The base rate of glass breakage is so low and the incidents so random that you can't really form a coherent theory around it.
I can tell you because I have had my car's glass broken and nothing taken when I had things that both a homeless person would like (blankets, socks, warm clothes) and things a savvy criminal would like (expensive shoes, electronics, DSLR, GoPro 360). I've also had the glass broken when I have had nothing in the car and with it looking like it was both empty and had already been rifled.
People like to think that the actions they're taking are protecting them, but it's not those actions, that's just a rain dance type situation. The real determiner is where you park: indoors? safe. outdoors in an area with unsavory people? unsafe. outdoors in an area with few unsavory people? somewhat unsafe.