It's a 7mm acorn nut that holds the motor halves together.
If your mast doesn't go all the way up, it's broken. There are little plastic tooth pieces stuck in the motor you'll have to pull out. Fortunately, once you take that 7mm nut off, it will become incredibly obvious what's going on and what you have to do. It's all just sitting right there. Once you get all the old plastic shit out of the motor housing, you just reassemble it, and stick the new antenna in from the top, making sure the teeth face forward (or is it backward? It'll only feed one way, in any case).
Also make sure you have the right limiter collar on there. Most of them come with more than one, and the instructions will tell you which one to put on. It's srsly like a 10 minute job - I'd rate it at 1.5 difficulty stars out of 5 (cos there is one nut, after all).
Oh, and make sure you take that top collar off, too. I just used a pair of vise grips (very, very carefully, so as not to f*** up the paint on the fender) to take it off, but I understand AAP and AZ sell tools for this purpose.