Jetta 12V to 115V inverter wiring question

[hopkin]

I have a 12V to 115V power inverter from a Jetta (PN = 1K0907155A).
There are 3 wires on the 12V connection, brown (GND), White/Red stripe (12V) and a smaller blue wire coming from the car. (On the inverter side, 12V is solid red, GND is solid black and the blue wire becomes a yellow wire).

(I have the pigtail connector that plugs into the invertor, but not a picture of it )

You can see the red, black and yellow wires in the picture below. My question is what does anyone know what the yellow wire (blue on the car side) is for?

[73sports]

Not 100% sure but I think it would be for night lighting, when the cars just had a 12v lighter socket back there, they had a ring around the socket that lit up with the dash lights.
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[hopkin]

Thanks 73sports that makes sense, there is an indicator just above the socket: