The wiring grommet that comes with the power antenna fits the factory hole in the kick panel sheet metal.
The top of the power antenna is held by the fender.
A metal bracket was made to hold the bottom of the power antenna. It goes over these two door hinge bolts that are sticking out, and then I put nuts on them to hold the bracket in place. (hinge bolts marked with red arrows in pic below. Not my car in this pic, just grabbed this off the internet to show the door hinge bolts) The metal bracket goes forward from these hinge bolts, and down to the bottom of the antenna, where it bends horizontal to fasten to the bottom of the antenna.

Next the bracket was painted Plum Crazy to match the car, and everything was installed.