[zombieking]

1.Create A variable like Unused[carid] or something like that
2.Create a timer of your choice , the time will be in how much time the vehicle becomes "unused"
3.In the timer function , set the unused variable to 1.
4.Go to OnPlayerExitVehicle and call the timer here.
5.Make command that will respawn all the vehicles that are unused

So like that vehicles without a driver will become unused after some time ( time defined by the timer )