On all my cars that had the power antenna, the whole power antenna unit was
bolted to the frame of the car in the trunk. Go into the trunk, pull back
the liner and it will be there usually with a little bolt or nut holding it
onto the body. Unscrew the nut on the outside at the bottom of the antenna,
then unbolt it from inside the trunk. Then unplug the black antenna wire
(from your radio) and the power wires. Do the opposite for your new one.
That should do it unless I am forgetting anything.