As the company that financed you car, Nissan has the first lien on the car. There is no way that the car mechanic would be able to place a lien on the vehicle given the fact that you still owe Nissan a balance. What you owe to the mechanic is separate from what you owe Nissan. You cannot club both the amounts together. Even if Nissan did write off the account as a loss, they still have the authority to acquire the asset and will keep trying to do so.
If you wish to keep the car, then you should get in touch with Nissan immediately to discuss your payment options before they actually do repossess the car. It is more likely that they will continue with the loan if you can reach a current status on your past due payments. Believe it or not, they do not want your car; they want the money and will be more than willing to deal with you. They generally lose money on repossession. Once a vehicle is repossessed, the vehicle is auctioned, any remaining balance is charged to the debtor. This includes the cost of the repossession. In all likelihood, you will end up paying more money if you let them repossess the car right now.
As far as the mechanic is concerned, he should not have a problem to release the car once you pay the entire amount that is due to him. If you cannot afford to pay the amount in lump sum, you should try to reach a payment agreement with him as well.
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