In Spain you'll listen the three cases at once and all of them are perfectly valid.
-ito it's almost the universal way everywhere in the Hispanic world.
-ico it's widely used in the South of Navarre and Aragón and everyone will understand you. Heck, it's the diminutive from used by the hick people, and thus, it's uber known, altough you might look like a bumfuck village redneck sheepherd with a beret by using -ico outside of Navarre/Aragón.
-illo it's more from the South, but, again, understood everywhere.
In Argentina everyone will understand you, but if you don't use "ito" then people may ask where are you from.
"ico" is used in many countries of Central America and Caribe. I asked someone from Colombia, so I'm sure about Colombia but I'm no sure about every other country.
Is "illo" used in Madrid? I think I heard it in movies or TV programs from Spain.
In Argentina: "autito"
In Colombia: "autico"
In Spain: "autillo"
the same rule applies for all words, not only for cars.