factory hitch harness wiring (what is this wire?)

[hawkcharmer]

I picked up a factory hitch and wiring harness from a salvage yard. It came off of a limited and I have a base model. I connected harness to brake light, etc.
but I cant figure out what this one remaining (green/yellow) wire is.... I'm holding it in the picture below and it comes out of small black box

Any feedback is greatly appreciated.

Thank you