Lights on the back of the cab are just work lamps for when you are coupling a trailer at night or checking on the reefer unit. They can be red, amber, clear, purple, blue, etc. If the lights are purple or blue you usually can't run with them on, however I never heard of anyone being pulled over for that, except on cars and pickups( not commercial motor vehicles). My first tractor had clear lenses, and my second had red ones. I unhooked them when I installed the headache rack and wired my work lamps on the rack into that cicuit. If the tractor has a headache rack with lights on it ( the rack) then the lights will some times be wired in as turn signals. Normally OEM will have the turn signals mounted either inboard or outboard of the frame rail, all the way at the rear, near the mud flap brackets. Sometimes truckers will add a/m mud flap hangers with turn signal/running lights in it. There are after market light panels that are installed between the frame rails at the rear that have several marker lights and turn signals in them. DOT requires the red and white reflective tape (or just red) to be place at the top corners and down the side on the back of the tractor cab. If the tractor has a headache rack installed then the rack must have the reflective tape.
U.S. truckers, especially independent owner operators, take great pride in their rigs. They add tons of chrome accessories and as many lights as they can. It is a lot of work to keep up with it all but when you see a Pete rolling down the road that has all the chrome and lights, it looks awesome.
this is a center rear light panel with the turn signals being the big ones on the left and right.
Wade