How to replace Power antenna mast?

I have a 1996 Mercury Sable LS. The antenna is on the driver side rear of the car. The antenna won't go up or down although I can hear the motor running. I bought a replacement mast on ebay but I can't figure out how to take the existing one out. I took off that top nut thing (it looks like it needs a special tool, has 4 notches in it, but I got it off with a hammer and punch). Now what do I do to get the antenna out?
I pulled on it but it still seems to be in there pretty good.