Got my "EBay" antenna this weekend. The "slider part" serves no propose. It's just plastic. Maybe the antenna is manufactured for different models and another model uses the piece. I used diagonal wire cutters to cut it lengthwise. Then carefully pulled it off the antenna. It's two pieces, glued together just above the thin metal ring and easy to separate for cutting.

It works better than stock and as good as the 17" aftermarket I bought from AutoZone. I like that it can telescoped down to a shorter length and it looks a little bit cleaner.