See pic, thank you

 

Looks like it's never been used. Probably for something not on your car.

 

Do you have a rear heater behind driver seat.?
If not then this is the plug for that valve that is mounted behind the air-conditioner tank along the wall.

 

The "receiver" isn't your receiver/drier and it looks like you low-pressor sensor is hooked up. So if you cannot find a plug-in for it, it may just be an accessory plug that you vehicle doesn't need it for. Most cars come with looms for parts that are not found on some vehicles. Yours might be one. I'd look all around myself.