The safe voltage of the panel will depend on the current output. Most commercial 12v panels seem to output ~19v in full sun, at ideal angles, which is fine at low currents (also typical of the size of commercial panels that would fit in/on a car) since you're unlikely to raise the system voltage significantly when the vehicle is operating/drawing power.
For an idea of electrical demands of a small car (Pontiac Firefly / Geo Metro), see this thread:
http://ecomodder.com/forum/showthrea...olts-8908.html
And you're probably aware that if your panel is connected (however indirectly) to the battery when the vehicle is
off, you'll want a charge controller to prevent overcharging.