Many drivers have air fresheners, dice or a DMV-issued handicapped parking placard dangling from their front center rear view mirror. If those objects are too large, they can get you into trouble. California law prevents you from placing any object in the vehicle that impedes your view through the windshield. Plus, you have to have an unimpeded rear view of the highway for at least 200 feet behind your car.
TL;DR (Too Long; Didn't Read)
You must have at least two rear view mirrors, located so that you have an unimpeded rear view of the highway for at least 200 feet behind your car.
Number and Location of Rear View Mirrors
Under the California Vehicle Code, every California-registered vehicle except for motorcycles needs at least two rear view mirrors. One must be located on the vehicle's left-hand side; the other can be located anywhere, as long as you have a clear view of the highway for at least 200 feet behind your car through both mirrors. Foreign-registered vehicles and motorcycles must have at least one rear view mirror meeting the 200-foot standard, including one affixed to the left-hand side of the vehicle.
Vehicles With Restricted Rear Views
Where there's no rear windshield or you're towing another vehicle, you must have two rear view mirrors on both the left- and right-hand sides of the vehicle in lieu of a front center rear view mirror. The same applies if you've loaded the vehicle in such a way that your view to the rear is blocked. Passengers don't count as an obstruction even if they temporarily restrict your rear view.
Placing Objects on Rear View Mirrors
In California, it's unlawful to drive a car with any object placed or installed on the vehicle that obstructs or reduces your clear view through the windshield or side windows. While there is an exception for objects dangling from rear view mirrors, the test is whether the object impedes your clear view of the highway. You can be stopped and cited if it's objectively reasonable for a law enforcement officer to believe that the object is obstructing your view in any way.
Penalties for Violation
Violating the rear view mirror provisions of the California Vehicle Code is an infraction punishable by fine only. Generally, it will not add any points to your license. The base fine is $25, but each county adds penalty assessments that vary among jurisdictions. The fine is likely to be about $200 with your county's added fees – for help with fee explanations, contact the county court listed on your citation. The California Vehicle Code adds a 50-percent late charge for any traffic penalties not paid within 20 days. An additional consequence of being cited is that you might find that your insurance company raises your driver's insurance premiums, as well.