For receiving, or for transmitting? Since this is rec.radio.scanner,
you are probably concerned with using the antenna for receiving. I feel
the urge to comment anyway.

For receiving, you're probably OK, but for transmitting, I think you
might have problems with this. The average mobile antenna is designed
to operate while attached to the big, metal plane that is the roof of a
car. A similar question often arises at the Usenet group
rec.autos.makers.jeep+willys. Jeeps do not have a metal roof, or
(generally) any roof at all, so antennas end up mounted on the rear
bumper.

You might do well to pay a visit to that group and ask there, as Jeep
owners have some experience with the concept. Search both that group
and Google for the word "counterpoise."
http://groups-beta.google.com/group/...p+willys?hl=en