First, when you removed the old antenna did any of the old tail come out? If it did was it toothed or not? If nothing came out, you definitely have broken bits inside the motor you will need to remove.
When I was replacing the mast on my 300TE I couldn't get the tail to "take". After taking the motor assembly apart (again) I realized the tail was just getting caught on a little lip in the tube when I pushed it in and wasn't going in far enough. The trick was to bend the tail straight before inserting it so it would go down all the way to the gear.
Assuming it's a Hirschmann antenna, getting the cover off is easy. You'll probably need to remove the antenna from the car first. One side of the cover snaps on. Just use a screwdriver to carefully pry the snaps around the edge. You may find you have other broken parts when you open it up. I had a broken gear and belt. Fixed the gear myself and got a new belt from
www.antennaworld.com.
Good luck.