The wire for the power antenna should be white/black. i think what the radio does is supplies power to it when it is on this makes the antenna extend, cut power to it and the antenna should retract.
all you would need to do is rig up a switch between the antenna wire and +V form the car acc, thus when the switch is off the antenna should retract and when it is on it should extend.
this is all going on the theory that it extends when there is power supplid to the wire.
hope that helps, Tom