How do trailers attached to trucks or cars also have tail lights that correspond to the vehicle pulling it, since it can be removed?

Question

Driving in a suburban, part rural town and on the interstate, I see trailers being pulled by trucks and cars that have the brake lights go off when the vehicle’s lights go off. But the trailer doesn’t come with the car or truck when people buy it new, so how can any trailer work with any vehicle that’s hauling it?

I’m a normie, by the way. I’ve never driven a truck, nor ever hauled a trailer, so if my assumption of “any trailer can work with any vehicle” is wrong, please correct me!
Thanks for your answer!

In: Engineering

Answer 1

Trailers have wiring harnesses which stretch all the way from the back to the front of the trailer, which can plug into any tow-capable vehicle. At the the front of the trailer, the harness has a unique plug which fits an outlet wired into to the truck. It is about as simple as plugging in a lamp to an outlet in your home. Once plugged in, anytime the tow vehicle’s own tail lights are engaged (braking, turn signals), the same signals are activated on the trailer.