I just purchased the OEM mast and installed it. If you can hear the motor whirring when the radio is turned on and off, chances are your motor is fine, you just need a new mast. The ones off ebay work wonderfully and are about $15 for a new mast.

To install it, just remove that metal thing holding the assembly on, pull the old mast out with the radio on, and then put the new one in and turn the radio off. Make sure the teeth are facing toward the back of the car when you put it in. Put the metal thingy back on and you are set.