I can not speak for antennas used in Cell telephones but all antennas placed on vehicles especially those for AM are compromises. If you want an optimal Am antenna cut for the middle of the AM band lets say 790Khz, it would have to be 592 feet long. Hardly practical for vehicles. Appropriate electronics are built into antennas on our Zeds so what you are seeing is the very tip of the antenna and I really doubt that shorting one slightly will make that much difference on AM. But if you like to listen to a station that is far away, no matter what, the reception will be worse than if you listen to local stations. So your satisfaction with an antenna depends on the locality of where you are listening more than the length of the antenna on the car. 
