Mine was doing the same thing when I bought the car. I got a new mast from eBay. Click here
Antenna Mast
It will come with instructions as well. When I tore mine apart there was a piece of the white plastic with teeth on it broken off in the bottom of the antenna. Just compare the legnth of the new antenna with the old one, If the new one is longer you can pull the cover off the bottom of the antenna motor and pull the piece out. The same company also offers a complete power antenna assembly, there is a link to it on the ebay page.
I wouldn't go to the junkyard due to the fact that these are plastic and will break eventually. You're only looking at 20 bucks for a new one.
