Not having a back window could easily give the appearance that the car is junked/abandoned - EVEN IF you are still driving it. Every city/county has their own wording in their ordinances about what does or doesn't constitute "junked or abandoned" - you'll want to find out for sure what the ordinance that applies to your particular jurisdiction says; as it's likely a prime target for tow. (The reason being that lots of cars with missing windows tend to be vehicles that have been stolen; many of which end up dumped in apartment complexes. Allowing vehicles without windows to remain, even if driveable, serves as a red flag that "hey, this is a good place to dump such vehicles because they just let them sit - no one bothers to check on them or do anything about them" - which is exactly what complexes who tow such vechicles are trying to avoid. Not coincidently, it falls under a theory of crime prevention actually CALLED "Broken Windows Theory".)