Well, it is the same problem for Columbo, Tatort and other similar series made like lots of "TV movies" though they could be considered as a series with episodes numbers.
If several cars come back in each "episode" it may be more efficient to list it as a series.
On the other hand, if the episodes are really different then you could list it like IMDB does (except if you prefer to list it like a series anyway).
About the IMDB link, if you list it as a series you could use the first "episode" as IMDB reference.