Gauge cluster wiring

[TnT4x4]

I am replacing my dash and am reconnecting the gauge cluster. I noticed there is a third port that I can't find a plug for. I can't remember if it had one plugged in it. Can someone help with this please, thanks

 

[mrack]

Look in the ports for dust. If the port in question is dusty and the rest are clean, no connector goes there.

 

[TnT4x4]

That is the troubling thing, it was clean. But I could not find a plug, so I'm looking for some clue as to where to look. Thought that it might be a test or calibration port?