Motor vehicles not covered by this part are:

(a) Military design motor vehicles;

(b) Motor vehicles used for military field training, combat, or tactical purposes;

(c) Motor vehicles used principally within the confines of a regularly established military post, camp, or depot; and

(d) Motor vehicles regularly used by an agency to perform investigative, law enforcement, or intelligence duties, if the head of the agency determines that exclusive control of the vehicle is essential for effective performance of duties, although such vehicles are subject to subpart D and subpart J of this part.