I've tried this and the magic number was acutaly 105mph.
The connection is likely from a different car using a similar wire harness, or a carry over from an option overseas that wasent offered in N/A. It's been there since the first 3rd gen.
i'm thinking it does have something to do with that also, which is why you can not see it when it's connected. so OP test your squirters to confirm or deny.
I'd have to say it's either fog light or washer pump. Nothing else should even be there. I don't have it on mine unless it is under the sheet metal which I know there is fog light harnesses just not sure what they look like since I never studied those in the 7 or so times I've had cover off.
Mine wasent Pluged in and everything worked for me... It's just an extra connector.. Abd it's not uncommon for manufactures to take looms from othe cars and fit them to new ones or incorporate extras for future developments. I'm sure no one will be able to answer the question short of mazda