[PowderMonkey]

“Chocolate cars”? First time I’ve heard it. Idiomatic expression. Direct translation would result in confusion. Example: “A dirty old man” who chases after young girls, said in Spanish would be “viejo verde” (green old man). We use the term “grey market” (questionable) vehicles to denote any vehicle brought into the U.S. by anybody other than the manufacturer.

[SJSAMPLE]

Exactly. I’ve asked my Mexican counterparts to explain, and they’ve confirmed that “chocolate” refers to the candy and isn’t some misundersanding or interpretation diffuculty. However, they’ve been unable to explain either the origin or the etymological reference to actual chocolate.

[SJSAMPLE]

I should have thought to Google this before;

"But Amda argues that opening the border at a time when the country is struggling to stem a huge inflow and circulation of illegal cars, popularly referred to as "coches chocolate" [chocolate cars], would be ruinous for the industry"

¿Chocolates? Yes, the Mexicans love their illegal, smuggled cars so much they call them "chocolates."

This article is particularly informative;
http://lang-8.com/27649/journals/981794/%2522Autos-chutos%2522-%253D-%2522autos-chocolate%2522%253F