Pretty sure the control for it comes straight from the radio, but most likely it's the power antenna motor/mechanism itself, or the mast has rusted internally.
It's accessed by pulling the wheel and wheel well liner, then there is a bolt on the frame of it in the wheel well area, and a nut up where it goes through the fender, probably under a plastic trim piece, carefully popped off so you don't break tabs on it. There might not be a bolt on the bottom to anything if it has already been replaced with an aftermarket unit.
I don't know if the gear housing on it is permanently sealed or if you could pop it open and add some light grease, but once you have access to it you can apply 12V to the power leads to see if it works acceptably.
I'd replace a power antenna with a fixed mast type because I don't have a problem with vandals nor do I go through automatic car washes much (though I have small vice grips in my glovebox that I could take an antenna mast off with if necessary).