As a former glazier, I’m aware that all side windows of automobiles are made of tempered glass (baked in an oven) and are weakest on the edges. To break one, simply insert a screwdriver or knife between the window frame and glass and pry! The glass will shatter quietly.
If you’re breaking the window to get to a baby in a car seat accidentally locked inside, go to the opposite side to minimize any danger to the child.