The antenna base is removed by removing the retainer on the top. Grip it with a large pair of pliers or a wrench and turn counter-clockwise.  Note that the mast doesn't have to be removed to do this.  That's the reason I was replacing the antenna!

Remove the antenna wire from the radio and pull it out from under the dash.  Tie a string near the end of the wire.  This string will be used to pull the new wire into the car.

Step 3: Remove The Old Antenna

Pull the antenna and wire out with the string attached.

Tie the string to the end of the wire on the new antenna.  Note - Try to tie the string as close the end as possible as the hole the wire goes through is small and there really isn't room to have the wire go through when it's doubled up.

Use the string inside the car to pull the antenna wire through the hole and into the car.  Note - Considering the wire is L shaped near the end I was able to drop the wire into the fender and locate the hole without using the string although I had the string ready as a backup.

The base can be installed without removing the fender.  Loosely attach the upper and lower retainers and slip the lower one into the hole in the fender.  With some maneuvering the entire lower portion can be coerced into the fender.  Adjust the antenna base such that it is situated properly and then tighten the upper retainer.

Run the wire back under the dash and plug it into the radio.