From what we have read it appears that New Jersey is concerned with the front and side windows but not so much with the rear window. For example the New Jersey Driver's Manual states that a motor vehicle windshield should not have chips or cracks in it. It should always be clean inside and out but does not mention the back window at all. Also New Jersey law prohibits most drivers from adding tint to windshields and front-side windows but again does not mention the rear window.

The law that would appear to be the one that should list rear windows if you could be ticketed for covering it or not having the glass repaired right away would be New Jersey Statute 39:3-74. This section of NJ law is titled Windshields must be unobstructed and equipped with cleaners and states:

Every motor vehicle having a windshield shall be equipped with at least one device in good working order for cleaning rain, snow or other moisture from the windshield so as to provide clear vision for the driver, and all such devices shall be so constructed and installed as to be operated or controlled by the driver.

No person shall drive any motor vehicle with any sign, poster, sticker or other non-transparent material upon the front windshield, wings, deflectors, side shields, corner lights adjoining windshield or front side windows of such vehicle other than a certificate or other article required to be so displayed by statute or by regulations of the commissioner.

No person shall drive any vehicle so constructed, equipped or loaded as to unduly interfere with the driver's vision to the front and to the sides.

So the rear windshield and glass is not discussed in this obstruction NJ law which would normally be the one police could write you for say a distorted front windshield, dice hanging from a rear-view mirror, a GPS being placed on inside of the front windshield.

There is also New Jersey Statute Title 39 subsection 3-75 (NJS 39:3-75) is titled Safety Glass. Here it states that among other things that:

No person shall drive any motor vehicle manufactured on or after July first, nineteen hundred and thirty-five and registered in this State unless such vehicle is equipped with approved safety glazing material wherever glazing is used in doors, windows and windshields. The term "windshield" shall be construed to include wings, deflectors and side shields; also front corner lights adjoining windshields.

Every section of safety glazing material shall be legibly and permanently marked with the manufacturers' distinctive designations, under which the safety glazing material was approved, so as to be visible when installed.

No person shall drive any motor vehicle equipped with safety glazing material which causes undue or unsafe distortion of visibility or equipped with unduly fractured, discolored or deteriorated safety glazing material, and the director may revoke the registration of any such vehicle.

So the definition for windshields here does not list rear windshields again and thus it would not appear that you could be cited for your rear windshield issue. However since police sometimes tickets things up under their own discretion and the judge looks at the law in their own way so we would advise you to check with the local police department to make certain there is not a section of NJ law that they could cite you under while you have your back window covered with the clear plastic and not a real safety glass rear windshield.