Looks like 26708.(b).8: (bolding is mine)
26708.
(a) (1) A person shall not drive any motor vehicle with any
object or material placed, displayed, installed, affixed, or applied upon the windshield or side or
rear windows.
(b) This section does
not apply to any of the following:
(8)
The rear window or windows, if the motor vehicle is equipped with outside mirrors on both the left- and right-hand sides of the vehicle that are so located as to reflect to the driver a view of the highway through each mirror for a distance of at least 200 feet to the rear of the vehicle.
Many of the automotive tinting companies highlight a CVC requirement that says if you tint the back window, you'll need to have both outside rearview mirrors but they don't specifically mention the pertinent CVC clause.
ref:
Today's Law As Amended