Thanks for this very helpful thread. The excellent photo posted by Jerry Bransford makes it really clear. I would like to emphasize something in the post by TJDave: "You can use a little flat head screw driver to pry it out....if you don't want to mess up the rubber part of the knob...warm it up with a heat gun or hair drier. This will soften the rubber."
I tried to pry my cap out in 40 degree weather, but it was not coming out. I did not want to force it and cause damage. So I used a hair dryer on it for a couple of minutes to warm it up. This worked like a charm. It is not the plastic center cap that softens; it is the rubber knob itself. So you can focus the heat on that part. Then the circular edge of the knob becomes soft enough to easily pry out the center cap without damage.