The courtesy lights are switched ground with a constant 12V+ provided by the "consumer switched off" circuit. Take a look at the wiring diagram and then confirm with a multimeter. It is right there in black and white. Any brown wire, solid or striped, is ground.
Pull out one of the door lights and test it. You will see.
You can do it with relays, but not with diodes.
Here is another viewpoint, showing the inside light (dome light) switching function (inside the light fixture). If it had a switched positive, it would not work correctly. When you switch to pin 3, the light stays on always because it already has a constant 12V+ and you are switching it to ground. When you switch to pin 1, it is getting switched ground from the immobilizer.
The positive is constant and supplied by the "consumer switched off" circuit, which stays active with ignition off to provide power to the courtesy lights, mirror and seat adjust, etc.
Of course, I suggest that you go and put a multimeter on the door light and see for yourself. It takes all of a minute.
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