(a) Buses. Every bus shall have a safe, effective defroster and heating system, which shall produce sufficient heat to provide reasonable comfort for occupants. All heating system pipes and radiators shall be shielded to protect the occupants and their clothing, and the moving parts of all heaters and defrosters shall present no hazard to occupants. Air intakes shall be located to minimize the entrance of exhaust fumes into the bus. If combustion heaters are used, they shall be installed on new buses by the body or bus manufacturer, and on buses now in service, by authorized dealers or garages. Heaters and defrosters are not required for trailer-buses of open air construction.

(b) School Buses. Every school bus shall be equipped with an effective defrosting device of the hot air duct type.

(c) Trucks and Farm Labor Vehicles. Every truck and farm labor vehicle shall be equipped with an adequate mechanically operated defrosting device, or adequate air-circulating system that removes snow, ice, frost, fog, or internal moisture from the windshield.