Ariz. Rev. Stat. § 28-4010 – Motor vehicle liability policy; primary and excess coverage

A. If two or more policies affording valid and collectible motor vehicle liability insurance apply to the same motor vehicle that is involved in an occurrence out of which a liability loss arises and one of the policies affords coverage to a named insured engaged in the business of selling, repairing, servicing, delivering, testing, road testing, parking or storing motor vehicles, both of the following are conclusively presumed:
1. 1. If at the time of loss the motor vehicle is being operated by a person engaged in one of the businesses or by the person's employee or agent, the insurance afforded by the policy issued to the person engaged in the business is primary and the insurance afforded by any other policy is excess.
2. 2. If at the time of loss the motor vehicle is being operated by a person other than a person described in this subsection, the insurance afforded by the policy issued to a person engaged in a business described in this subsection is excess over all other insurance available to the operator as a named insured or otherwise.
B. Except as provided in subsection A, if two or more policies affording valid and collectible liability insurance apply to the same motor vehicle that is involved in an occurrence out of which a liability loss arises, it is conclusively presumed that the insurance afforded by that policy in which the motor vehicle is described or rated as an owned automobile is primary and the insurance afforded by any other policy or policies is excess.
C. The presumptions stated in subsection A may be modified or amended only by a written agreement signed by all insurers who have issued a policy or policies applicable to a loss described in subsection A and all named insureds under these policies.