Sure they are, as long as there's a car to be listed it can be listed, it doesnt matter if in a whole serie that lasted many years there's only one or two cars... this is a good example:

http://www.imcdb.org/movie.php?id=192877

Those are the only cars that appeared in that series that lasted from 1999 to 2006, but if there are cars they should be listed... doesnt matter if there a just two or three and the series lasted ten years...