You said it wouldn't be possible to allow players to organize their vehicles like they can with items in houses, but what if there was a set vehicle priority for everyone? You could implement some code into the server that would record whenever someone pulls out a vehicle, and count how many times each vehicle gets pulled out. After a couple days, you would have enough data for a distribution of how often different vehicles were used. Then, you could order everyone's vehicles in their garages to match that distribution. Hatchbacks first, etc.
If not this, at least put hatchbacks at the top...