Cal. Veh. Code § 27907
There shall be displayed in a conspicuous place on both the right and left side of a tow truck, a repossessor's tow vehicle, or an automobile dismantler's tow vehicle used to tow or carry vehicles a sign showing the name of the company or the owner or operator of the tow truck or tow vehicle. The sign shall also contain the business address and telephone number of the owner or driver. The letters and numbers of the sign shall not be less than 2 inches in height and shall be in contrast to the color of the background upon which they are placed.
A person licensed as a repossession agency pursuant to Chapter 11 (commencing with Section 7500) of Division 3 of the Business and Professions Code, or a registrant of the agency, may use the license number issued to the agency by the Department of Consumer Affairs in lieu of a name, business address, and telephone number.
Ca. Veh. Code § 27907