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Is there some sort of switch at the door jambs? On previous cars I've owned there's usually some sorta door jam switch where the door is closed and it presses in the button at the time. when the door is opened the button on the switch pops out automatically so it then basically turns on the lights.
It's the driver's door switch. If the passenger side comes on, the bulb & power have to be OK.
You're correct, on lesser cars there is a simple plunger switch that provides a ground for the light circuit when the door is opened. However, at least on my 2002 GS300, it's not that simple. The light comes on when the door is unlocked, not opened and as far as I can see there isn't an external plunger switch anywhere.
I'm going to guess that the light is controlled by a switch inside the door that's driven by the locking mechanism.