in a sunny day, shooting a shiny car, how do I want my picture? it may be the first question we should ask.
Before that, we should understand there are 3 type of reflection : diffuse reflection,
direct reflection and glare (polarized reflection).
Any light can produce any reflection, depending on the type of subject, and the angle of the light source related to the the camera, if the light source came within the " angle of family" (in short, the angle of incidence equals the angle of reflectance. search the web for more detail)you got maximum direct reflection and glare.
polarized reflection cannot be as bright as an unpolarized direct reflection.
Back to the question, if you want the shiny car LOOK shiny, and rice of reflection of the environment:
Look for the "family of Angle" related to the light source (the sun), set up your camera there.
metering only on the "body" of the car, under 1/3 EV
Polarizing filter is an option, depend on if there is too much polarized reflection.
If you DON'T want the car look so shiny:
stay out of the "angle of family", in this case it is really hard for this subject (a car), but at least the part you are focusing on.
use Polarizing filter, cut the polarized reflection as much as you can.
wait for the cloud to block the sun. A "giant soft box" will help the most.