This has been a problem for two-way radios on ambulances with fiberglass rears. On those the frequency is higher and easier to do with metal plates or 4 wires going out horizontally 90 degrees apart. It's difficult to get enough ground plane for this frequency, especially AM, FM band is a little easier. I would run a 1 inch wide or larger ground braid from the antenna mount, (ground), to the closest metal part of the body, shouldn't be that far. Try it and see what happens. Depending on the radio you sometimes have to tune the antenna at the radio for maximum signal, usually a small trimmer near the antenna connector, sometimes it's behind one of the knobs.