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Transistor
It is simply a electronic switch, the 1K resistor is a current limiting device to protect the transistors from drawing too much current. It would help if the writer listed the EBC codes on the drawing. I will try to open this up a bit. The 1K resistors go to the Base of the transistor, it looks like the Emitter is grounded and the Collector supplies switched power to the harness. The Arduino can only handle a few milliamps of current so the transistors are used like electronic relays to trip the regular relay in the car.
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