Have you looked at the antenna (inside the trunk) to see if you can visually tell what's wrong?
Pull back the trunk liner on the passenger side, and look. If you hear the noise of the motor running, you've eliminated a HUGE list of possibilities, already! You obviously have power and a good ground, etc. Now you're only left with the motor, the gear, or the connections between the two.
Take a look at that antenna base in the trunk and see if anything's hanging, etc. Have a friend turn the radio off then on while you watch the motion. Perhaps it will be something you can reattach, or fix.
