From your description, and the one in the manual that says "remove the waterproof cover of...", I think I located it. It IS where the photo with the arrow indicates, but it's wrapped tightly with some other wiring.

This is actually a photo-copy of the cover I took off (I don't have a digital camera) and there IS only one prong inside the connector. The cover has definite seals inside it for water protection.

scan0050.jpg