Really difficult to not advertise here… 
My script (AdvancedParking) can save (in theory, would need to be implemented separately) every single vehicle on your server. (Though I would argue that this could impact performance significantly)
There is a client side export that allows to save any vehicle. This could be implemented by using the entityCreated event and triggering a client event from there.
By default AdvancedParking saves any vehicle a player has entered/left (including NPC vehicles if it’s not disabled via config).