most of the people in the anime scene are JDM car culture enthusiast.

well that news to me (though I'm sure there is
some crossover)
It seems like an interesting idea…
however… getting an event into Fanime will likely take more then posting a rough outline of what should be involved. This would have to be coordinated with a Fire Marshall (as you said), the convention facilities, the people showing cars, and might have special staffing requirements, also might involve legal issues Fanime will have to address so they can't be held liable if something happens to any of the vehicles (the same way they aren't liable if something happens to art in the artshow). The best way to get an event to happen is probably offering to coordinate with Fanime and run the event. Even then you would probably need to have experience running similar shows or develop or rapport with Fanime staff (by volunteering or becoming a staff member). Otherwise it will mean someone else will have to coordinate and run the event on top of what they already do to support Fanime. It seams to me that new programming that happens at Fanime tends to be something that a staff member is already familiar with and has an interest in running. So unless you can get really wide fan support for this, or a staff member willing to adopt this as a project, I think the odds of this happening are slim. Not to mention they already have the swap meet is South Hall on Day 0 and Day 1.
(disclaimer: I am not staff and do not speak for staff. I'm just a long time Fanime attendee)