Dome light, can’t find diagram

I have two W123s at the moment and this dome light from the 84’ is nothing close to the 77’. It’s intermittently not working, so it’s getting power and has a good bulb.

Question is, does anyone have a diagram that has 4 pins? Everything I’ve found is 3 pins like the 77’. Second, why does this thing have a PCB? Assuming one pin is 12V constant, 12V from the door switch, and then the third is ground, what’s the fourth?

Thank you!

I have two W123s at the moment and this dome light from the 84’ is nothing close to the 77’. It’s intermittently not working, so it’s getting power and has a good bulb.

Question is, does anyone have a diagram that has 4 pins? Everything I’ve found is 3 pins like the 77’. Second, why does this thing have a PCB? Assuming one pin is 12V constant, 12V from the door switch, and then the third is ground, what’s the fourth?

Thank you!