The exterior whip antenna is more for FM reception than AM stations, the AM antenna is actually inside the headunit.
As for the stuck antenna, might try dribbling in some penetrating lubricant to breakup/loosen any corrosion in there. Then work the antenna back and forth until it pops free. Sometimes takes a considerable amount of force to extend the antenna. Once it frees and you are able to extend it all the way out, take that time to liberally coat the antenna with light machine oil or similar - basically anything that will "stick" to the antenna surface. Work the antenna back and forth make sure its entire path gets a good coating of oil, and reapply every month or so to keep it in good shape.