You shouldn't need to worry about the motor. Just unscrew the antenna, turn your radio on and pull it out. Feed the new one with the chain facing the motor (I dont remember which way this is). Turn on the radio and let it pull the new antenna in. Give it some resistance when you are down to 6 inches. The motor should stop, so now you can screw the antenna nut in. If you power cycle the radio, it should go all the way down.
In my case, my chain had broken and I had a part in the motor. When I opened the motor up, it was filthy. It might not be a bad idea to open the motor up and clean/relube it while the old antenna is out.
