Help identify this wire coming out of the back of a stock stereo

[GNVAIR]

I have 3 factory EQ head units. 1 is the 85-86 style with the extra chrome on the face.
The other 2 are the 87 style without the extra chrome.
2 of these stereos have an extra wire coming out of the same hole in the back that the orange and brown wires come out of. The wire is white with a black sheath over it and has a single female spade connector on the end.
Does anyone know what this would be for? The 85-86 stereo has it and only 1 of the 87 stereos has it.
Any info is appreciated.

 

[mikestertwo]

I'm not sure but I think it is on the ETR models. I think it is for dimming the display when the lights are turned on.