"storage" could mean several things: awaiting customs release, awaiting enough vehicles to fill a specific number of train cars, etc.
personally I believe its the latter of the two. the plant in Toluca Mexico also builds the Dodge Journey which comes to the States as well. my 'guess' is that the 500's and Journeys come to a holding area and are awaiting more vehicles to fill a train car to either go east, west, or north.
if you happen to be in an area that has not been getting many 500's because of a lack of Studios, you may have a longer wait.
again, pure speculation on my part......but it makes sense to me. I hope you all get your 500's soon!
