I'm running on memory here because the car's in storage for the winter, but IIRC Fred's got it about right.
Move the trunk liner concealing the antenna out of the way (two pressfit buttons at the front, if memory serves).
Remove the single bolt holding the bottom bracket of the antenna housing in place on the molding to which it attaches.
Remove the bolt holding the ground lead onto the car.
You should be able to remove the antenna from its usual position down into the trunk, pulling it down through the grommet.
Remove the grommet, pulling it outward, and insert a new one from the outside; I seem to recall using a pliers gently from the inside to help get it fully seated.
Reinstall the antenna reversing the above procedure.
Allowing for stupid time and fiddling with the grommet, you still shouldn't need more than maybe fifteen minutes.

__________________
Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)