I've had a real slow drip at my "AT Protection Switch", which is about where you described on my car. I just tightened it up with a pair of pliers. There are no flat sides to put a wrench on.
The switch has a plug on it that looks like a spark plug wire on a lawnmower. Ther wire comes off the sensor and makes a right turn with a "boot" on it.
Check it out.