49 CFR 392.7
No commercial motor vehicle shall be driven unless the driver thereof shall have satisfied himself/herself that the following parts and accessories are in good working order, nor shall any driver fail to use or make use of such parts and accessories when and as needed:
Service brakes, including trailer brake connections.
Parking (hand) brake.
Lighting devices and reflectors.
Windshield wiper or wipers.
Rear vision mirror or mirrors.
[33 FR 19732, Dec. 25, 1968, as amended at 60 FR 38746, July 28, 1995].