Radio Antenna

[Sidewinder]

My radio antenna doesn't quite work properly. I am assuming, that when the car is off and the radio is powered down, the antenna retracts fully into the car body. Mine sticks up about six inches. When the motor tries to extend or retract, it makes a horrible grinding noise. It looks like the entire antenna unit is slightly bent.

Is there a way to disconnect the motor so it does not try to extend the antenna without actually disconnecting the entire antenna? Also, are these units replaceable for fairly cheap? No more rebuilt transmission prices!