I don't know if those lights were optional factory equipment, but most were add-ons. They are required by either DOT or ICC regulations for trucks above a certain GVW rating. The add-on lights were often wired through the light switch, but in many cases, a light relay was necessary, or at least advisable, because of the lighting load. On large trucks, those lights were often wired through a separate switch and relay.
The typical arrangement for cab lights was/is three in a row at the center, front of the cab and one on each side, sort of at the edge of the windshield. In the past, a light bar with the center three was common, but later that gave way to three separate lamps in the center.
Harvester
