A good choice for vehicles without adequete ground plane would be the
!/2 wave end fed antenna that does NOT require any ground plane.

One such antenna he www.comtelco.net A1812A 44

Also can be orderd with the heavy duty .125 dia whip, The whip is about
39" tho.

The radiation pattern is the same as a 1/2 wave dipole and much the
same as the
1/4 wave whip.