There are a couple of options that you have. The first option is to create a grounding plane and mount it on your vehicle. You will need a piece of ferrous metal about 4” in diameter and a double stick adhesive. This will create a grounding plane so the signals can bounce into the antenna.
Another option would be to mount an antenna that has a grounding plane built into it. We would recommend the 4g Trucker mount antennas (304415 or 304414). These would require you to mount the antenna on a bracket attached to your vehicle.