Hi Rob,
Not that I am an expert here ...
I guess you are referring to an areal / antenna like the original Hirschmanns and Bosch units ...
Your observation is correct.
Usually there are three wires going to the antenna motor.
First one is constant current (permanent positive). Motor needs this for feed to drive the antenna out and in.
Second is "plus" coming from the radio which "tells" the motor by a relais-like mechanism to drive out the antenna.
If you turn off the radio this positive signal disappears and the relais-like stop-mechanism switches to the opposite pole which drives the motor (armature) the other direction to pull the antenna in.
Third one is ground.
That's what I remember ....
(hope I am right here)
Achim