It looks to be the plug for the passenger side vent when installing the ac like yours shows (as mentioned above). I have the CAA Perfect Fit AC and it has the heat/defrost/ac all in one. You have to close up the passenger side vent because you have no way to open/close that vent anymore with the ac component mounted (remember it replaced the heater and vent on that side, if you still have that installed in the car. The original heater box is gone.) I left my drivers side vent because it has a pull/push knob/cable to control it, if I am not using ac at the moment. But with the heater box gone, if you don't use that to seal the hole on that side, you would just have lots of outside air flooding the passenger side with no way to control it and lose a lot of ac cooling capacity.
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