Hi Richard
Chances are (not entirely sure without looking at a circuit diagram) but the aerial will probably be operated via a relay, hence the trigger wire (usually blue in colour from the stereo). Once the relay is triggered then the aerial will operate with a negative one side and a positive the other, which will then reverse when it is retracted.
In this case, if you put power to it directly (+ & -) then reverse it once it has elevated then it should retract. If not, then look for a relay as this may be faulty
Let us know how you get on with it
Cheers Trevor