Tough question..
Most states say as long as you have 2 side working mirrors, that rear view obstruction is fine.
But some states say the same, but have added that your vehicle has to be registered as a truck and have truck plates.
However in many states, any alteration to your vehicle are Illegal unless its oem equipped. ( so aftermarket radio, fender trim, rims, muffler tip, and everything else stupid) could get you a ticket.
Most likely where ever you look your not going to get a straight answer, and if you talk to the state and say you have some big woofers in your back seat, they will say its illegal even if theres no law.
I myself got pulled over a few times for illegal window tint in my old caddy with a wall (twice by state patrol, and once by local cop) and after i explained that the tint looks too dark because the wall is not allowing any natural light in and they looked at it they let me go without a ticket.
So im sure it all depends on the cop, and usally the cop is ALWAYS right!