California Code, Revenue and Taxation Code - RTC § 11156

A person shall, for the purposes provided for in Section 11154, be presumed to be operating a vehicle on the public highways only in the county in which he or she resides, or, in the case of other than a natural person, only in the county in which the vehicle is principally garaged, and he or she shall be subject to a local vehicle license fee surcharge only in that county.