Every author that cares to show a real world car in his work, tends to choose iconic ones - unless forced by product placement, that is.
Japanese cars are, evidently, not noticeable to the Japanese public so almost every hero in manga and anime has American or European cars, sometimes even at the cost of using a car that does not make much sense.
The attitude goes beyond Luoin III's FIAT 500 and Mercedes SSK.
In Neon Genesis Evangelion, Misato Katsuragi's car is an -unheard of - electric driven Alpine A310; Chihiro's parents' car use an Audi A4 Spirited Away, and in Yotsuba&! Torako drives a FIAT Panda 4x4 (the less reliable car produced in Europe in the late 90s) and Jumbo drives a Renault Kangoo...
Not to forget that Yakuzas seems to be unable to drive anything buit Mercedes.