Note that the orange cable has one purpose only. That's to allow for the constant +12 V feed in the socket. That and ground are the only "hardware only" things connected to the socket.
Note that what I say below is true for Volvo's TRM (TRailer Module). Other manufacturer's solutions may be different. Sometimes simplified and not as complete as Volvo's, to save money.
All other things pass the TRM. It's true that the TRM senses a connected trailer by the load on position lights output. As Volvo states, the original TRM requires a load equivalent to 10 W to consider a trailer to be connected. From 2013, there's a newer TRM version which also can sense trailers with more efficient LED taillights.
The socket standard has been augmented to include the unused pin 12 to be a trailer indicator. That is, if that pin is grounded, then that's the signal to the car that there's a trailer attached. But since most trailers don't support that funciton, the TRM doesn't require it either.
The absolutely simplest way to check if the car is "trailer aware" is to turn on the rear fog light. If the car has sensed the trailer, it will turn off the lamp on the car, and send the signal to the trailer only. This is to avoid reflections of the bright fog light from the front of the trailer. But it comes in very handy, since it makes it a simple one man's show to connect/disconnect and observe if the connection is recognized, as the rear fog light is right there, at the rear end of the car.
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