I have made rezzers in the past that rez a vehicle that I still own, but anyone can jump in and drive away. As soon as they drive away, another vehicle rezzes in its place. For garbage collection, I set the vehicles to delete themselves if the driver abandons the vehicle, and no one jumps in after a brief moment of time.
Most vehicles today will unsit avatars if they are not the owner. Look at your vehicle script, and remove the llUnsit method (make a copy of the script first as a backup before you make these changes). Then, have a friend try to drive the vehicle. If they are able to drive away, the you can call it a success.