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

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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
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There’s a little cable which runs between the truck and the trailer and can easily be disconnected when removing the trailer. This cable carries electricity and the brake light signal, which turns on and off the brake lights when the truck brakes.

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.

And adding to the previous comments, the connector is standardized, so all trucks and trailers hev the same connectors so its as easy as just plugging it in

After a truck driver backs under a trailer, he has to get out of the truck and perform the following:

*Hook up two (2) airlines between the truck and trailer that control the trailer brakes.

*Attach a 7-pin electrical connector that controls the trailer lights.

*Check and make sure the king pin is correctly seated in the fifth-wheel.

*Raise the landing gear of the trailer.

Another issue at times: if the trailer was loading from a loading dock using forklifts, it’s probable the rear tandems (the trailer wheels) have been slid to the rear of the trailer. This because a loaded forklift weighs sometimes as much as 15-20,000 pounds. If the tandems were left in travel position they would act like a fulcrum and the entire trailer would teeter-totter. So the driver also has to slide the rear tandems forward into travel position. This gives the tractor/trailer a much shorter wheelbase, thus a smaller turning radius.