Don't let anyone tell you you can't do it, I met a world traveler who drove a 69 VW van (a.k.a. Kombi) from California to Rio de Janeiro, via Argentina. If you are up for an adventure, you can do it. They took a ferry crossing somewhere in Central America, check out their webpage with more details: http://www.batequeixo.com/Batequeixo/Our_Trip_from_LA_to_Rio.html
Last but not least, beware of the existing colombian guerrillas, they hate Americans.
