How to take out the nipple
I recently replaced my radiator with a Behr OEM unit. It was immediately leaking from this nipple. (In fact, I thought it might be some sort of pressure relief valve) It turned out that the nipple had a crushed and torn rubber washer.
You will note that the nipple has a tri-pronged end that gets pushed into the radiator. It clips into place. In order to take out the "old" one, I put a 10mm socket into the upper radiator hose opening, and pushed it on to the "clipped" end to push it closed enough so that I could pry out the old one. I TIED A STRING TO THE SOCKET SO THAT IF I DROPPED THE SOCKET, IT WOULDN'T END UP AT THE BOTTOM OF THE RADIATOR. This worked flawlessly.