the aerial is just a metal stick.
Cut to an approximate size corresponding the a 1/4 or 3/8 of the 'wavelength' of the band its used on. bands are often call by the wavelength in the radio world, ie 70cm's is 450 mghz band, so you would 70*.25=18cm's etc
AM is just a fudge is the correct aerial length would be a little large on your car
And you can buy amped aerials, some cars have an amp included, some cars use the wires embedded in the rear screen (the heater) as the aerial and have an amp on that.