I have installed a radio in my syncro that transmits on
10, 11 (CB) and 12 meters.  After much pondering and
asking for advice, I installed the antenna on the roof
over the passenger cab area.  It is about 50" tall,
with a thin whip that can be easily removed, and that
is very flexible so it can whack into trees and such
without getting destroyed.  The antenna has an NMO
mount.  I drilled a 11/2" hole in the fiberglass and a
3/4" hole in the metal roof.  I ran the coax through
the right side pillar.  It is a very clean
installation, the antenna is not very visible it is so
thin and I can still use the luggage rack no problem.
I get 1:1 SWR at 27000 .

Last week I took a trip into the desert.  I tested the
radio and managed to get someone on the other side of a
10,000 foot mountain range (I was at about 1400 feet)
about 25 miles away.  It was my only real test attempt,
but the result was pretty good.  At the time, I was
about 2 hours from the nearest paved road and 21/2
hours from the nearest phone.  Very reassuring!

So, it appears that the roof mount provides a good
ground plane and was a good choice.  I will also be
able to seal everything up like it was never there if i
ever want to.