The main requirements for legal window tinting in California are:
1. The windshield and front driver's side and passenger's side windows cannot receive any aftermarket tinting.
2. If the rear window of a vehicle is tinted, the vehicle must have outside rearview mirrors on both sides.
The law prohibits any person from driving any motor vehicle with any object or material placed, displayed, installed, affixed, or applied upon the windshield or side or rear windows, with certain limited exceptions.
However, legislation signed into law effective January 1, 1999, exempts from the above prohibition specified clear, colorless, and transparent material that is installed, affixed, or applied to the front driver and passenger side windows for the specific purpose of reducing ultraviolet rays. If, as, or when this material becomes torn, bubbled or otherwise worn, it must be removed or replaced."
The above is from the California Highway Patrol's site where it discusses automotive window tinting in California