I am not sure I understand your issue.
You reaffirmed the debt and now wish to give back the vehicle. If you successfully reaffirmed and did not rescind the reaffirmation agreement in a timely fashion the debt was not discharged. If you fail to make the payments the lender can (and will) repo the vehicle and eventually it will sue you for the deficiency.
By reaffirming you re-obligated yourself to the debt and all of the consequences that go with it should you not fulfill your part of the contract.
Just FYI - the automatic stay ended, as it relates to you, upon entry of your Discharge. It was replaced by the permanent injunction of the Discharge Order which, in this case, appears not to apply to Santander.
Des.