The aerial may have snapped or stopped working in the past, and been replaced with a normal ie non electric one.
When wired correctly and working the electric aerial will be in the down position unless it has a live supply on the "trigger" wire, that is a output from the stereo that goes live only when the stereo is on, this can be a dedicated aerial feed, or a amp feed can also be used where amps are fitted, these usually have a remote turn on lead like the aerial.
I would firstly check that you actually have got a electric aerial, and that it has not been replaced.
If it is there and it has a permanent live to the aerial, as it should,when the radio is off, it should be down so could be broken, just to clarify the aerial will have a earth, a permanent live, AND the "trigger" live from the radio, the permanent live needs to be there so that when it loses the stereo live it has power to go down.
tabetha