A. If two or more policies affording valid and collectible automobile liability insurance apply to the same motor vehicle in an occurrence out of which a liability loss shall arise, and one of such 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 shall be conclusively presumed:
1. If, at the time of loss, the motor vehicle is being operated by any person engaged in any of such businesses, or by his employee or agent, the insurance afforded by the policy issued to the person engaged in such business shall be primary, and the insurance afforded by any other policy shall be excess.
2. If, at the time of loss, the motor vehicle is being operated by any person other than as described in paragraph 1, the insurance afforded by the policy issued to any person engaged in any of such businesses shall be excess over all other insurance available to such operator as a named insured or otherwise.
B. Except as provided in subsection A of this section, if two or more policies affording valid and collectible liability insurance apply to the same motor vehicle in an occurrence out of which a liability loss shall arise, it shall be conclusively presumed that the insurance afforded by that policy in which such motor vehicle is described or rated as an owned automobile shall be primary and the insurance afforded by any other policy or policies shall be excess.
C. The presumptions stated in subsection A of this section may be modified or amended only by written agreement signed by all insurers who have issued a policy or policies applicable to a loss described in such subsection and all named insureds under such policies.