Sounds like you're ebay buy has a broken/off the track plastic piece. There is a plastic cable which goes to the top of the antenna mast. It actually then attaches to the motor which makes the antenna go up and down. If you're getting noise from the motor when you should that would be my guess. You can take the antenna off the motor and find out. Not sure if you can buy this cable but I know when mine antenna broke I was told I could order just the mast and cable from Subaru. Seemed like too much of a hastle to me so I just bought a whole system from a junkyard. Hope you didn't pay too much.
Try here if you wanna go used again
www.car-part.com