Contact your local state Vehicle Commission (responsible for maintaining vehicle titles). See if your state allows an "Abandoned Title" for a vehicle. If so, title the vehicle in your name and use it.
However, I would suggest sending a letter (certified delivery) to both the lien holder (bank) and the last known address of the vehicle owner. In your letter, give them a reasonable amount of time (2 weeks) to contact you about their recoverying the vehicle. Point out that failure to do so within the time provided will allow you to charge 'storage charges' for the vehicle from the time that it was 'abandoned' and (coincidently), the storage costs meet or exceed any value left in the vehicle (this is to advise that their costs will exceed the value). Finally, advise them that if they fail to respond, you intend to obtain title to the vehicle to do with as you please without recourse. (These last few items are just formalities that could benefit you if their is any later claim to the vehicle).
An additional benefit of the letter to the former owner is, that it benefits you if he/she 'pops' up in the future with claims that you gave him 'open permission' to store the car and it was damaged or sold without his/her permission. Also, your notice will help prevent some states reviewing your allowing the storage as a bailment or a bailment for hire if there was any compensation involved.