So-called "no ground" antennas will work, but usually won't handle any real power. They're sold for boats and RVs. Pretty sure Fire-Stik has one.
What we'd do for a police car was a so-called 'ground-plane kit', made of metal-foil tape. Gets laid out as a square on the inside of the non-metal body surface, around the hole where the antenna mount is installed. It's a special tape with a conductive adhesive.
On 150 or 450 MHz, this isn't a very large area. Used this stuff on the inside of some fiberglass RV roofs, but gaining acess to a 9-foot by 9-foot area on the inside of the body panel won't be easy on the Fiero. Just isn't that large to start with. A ground wire isn't the same as a 'ground plane' that surrounds the antennas mounting point. A single wire won't mimic the metal surface area surrounding the antenna's feed point. That's what the foil does.
Never have tried putting a half-dozen or so quarter-wave long "radial wires" on the inside of a Fiero. The vertical antenna at an AM broadcast transmitter doesn't rely on the soil for a ground. They typically use 120 separate quarter-wave long wires in a circle radiating away from the tower's feed point. Makes for a more effective "ground plane". For a broadcaster, making the antenna efficient is money in the bank, so they'll spend what it takes.
If you're gonna run barefoot, a marine or motorcycle-type "no ground" antenna is the simplest solution. To use a ground-type antenna, the only question is how much metal you'll have to add to the vehicle to make that antenna work.
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