Yes that third wire is for the "park" function on the wiper motor. If your switch operates by supplying a ground source and you already have a constant 12v supply, I would think that the third wire would be a constant ground. Then you would have constant power to the wiper motor for the "park"function. This only powers the motor if the wiper is not in the "home" position and automatically stops the motor when it is.
“He who knows not and knows not that he knows not, he is a fool, shun him.
He who knows not and knows he knows not, he is simple, teach him.
He who knows and knows not he knows, he is asleep, wake him.
But he who knows, and knows he knows, he is wise, follow him.”
89 yj 4 banger 5sp. 2" BL 31x10.50 fixed up and sold
97 tj 4.0 Auto 4"RC lift 33x12.50 mud rovers on outlaw IIs . Smitty bumpers