It is our understanding that you can drive a car in California without back glass as long as it is safe in doing so and the failure to have a rear window does not impair the vision of the driver in any way. There are plenty of convertibles in CA that when their tops are down they do not have a working rear windshield.
California Vehicle Code Section 26710 is title defective windshields and rear windows. Here it states that it is unlawful to operate any motor vehicle upon a highway when the windshield or rear window is in such a defective condition as to impair the driver's vision either to the front or rear.
This law goes on to say that in the event any windshield or rear window fails to comply with this code the officer making the inspection shall direct the driver to make the windshield and rear window conform to the requirements of this code within 48 hours. The officer may also arrest the driver and give him notice to appear and further require the driver or the owner of the vehicle to produce in court satisfactory evidence that the windshield or rear window has been made to conform to the requirements of this code.
The law makes it appear that how it is upheld is somewhat up to the discretion of law enforcement officers in California and how they interpret this law about defective rear windows. So if you want to know about a specific situation where you would be driving a car without a rear window you should get clarification from local police or the California Highway Patrol about what is legal or illegal to do.
If your rear windshield needs replaced and you plan on making a Comprehensive auto insurance claim keep in mind that the insurer will want you to take reasonable steps to prevent further damage to your vehicle and thus keep rain and other damaging elements out of your vehicle. So if your back window was broken out and you plan on covering it with something such as cardboard or plastic to keep the elements out of your vehicle than whatever you put there may be considered an obstruction and could then get you cited by police for a defective rear window if you drove it.
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