If the groundplane as suggested doesn't do the trick, get an antenna
that's designed to be mounted on the rear window, rather than the roof.
That sort of antenna should be able to function properly without a
ground plane. Another tip for either kind of antenna is to move the
antenna in very small increments while watching the phone's signal
strength indicator. A distance of a few inches can make a big difference
when multipath propagation is present. If possible, find a general area
where the signal strength isn't overly sensitive to antenna placement.
If you can't, you either need to very carefully place it at a "sweet
spot" (and hope the spot doesn't move when the wind blows), or use a
directional antenna.

Roy Lewallen, W7EL