Most newer cars have and antishatter layer in the glass like the windshield so the glass will crack but still hold in place. You would need to brace yourself against something and push with your feet to get the glass to open enough to get out.
Somewhere in the rear of every vehicle are vents to allow air to escape when you shut the doors. This should help let water in the rear to allow it to sink level as well as drain holes in the bottom of the trunk.