At such time and under such conditions as may be authorized by the Safety Director or by the officer in command in the absence of the Safety Director, private cars may be used by the members of the Fire Department in the line of duty. When in use in the line of duty, such vehicles shall be considered to be fire vehicles and their drivers shall be considered immune from prosecution in those situations in which they would have been immune had they been driving a car or fire engine owned by the Municipality. When not used in the line of duty, drivers of private cars shall not operate or cause or allow to be operated any flashing or oscillating lights which might have been mounted on the cars in such position as to be seen by other motorists or pedestrians.
No such vehicle shall be equipped with a public address system, except when properly authorized.