I have heard it referring to both cars and people. The ways I have heard it a person can be a ricer, a car can be rice or a ricer car. The term ricer-burner is also used. Be careful where you use the term, it is not quite "pc." When I usually hear it, it is not used in a racist or discriminatory manner, but its origins can be considered racist so keep that in mind if you are going to use it. I really don't care, and don't take offense, but others might.
This is a great website for slang definitions, again not "pc." http://www.urbandictionary.com/define.php?term=Ricer
This is a great website for slang definitions, again not "pc." http://www.urbandictionary.com/define.php?term=Ricer