The answer to that question depends on Alabama law, and I am not admitted to practice in Alabama. Seek the advice of a local lawyer.
Speaking generally, in many states, including my home state of Kentucky, a secured lender like GMAC Finance can repossess the collateral if there is a default. They then have a duty to sell the collateral in a "commercially reasonable manner." This sale is often at a regularly scheduled auction.
Then they apply the proceeds of the sale to what is owed. If you still owe them money after applying the proceeds of the sale, in some states (including Kentucky) they can file suit against you for the "deficiency balance." The word "deficiency" here just refers to the fact that there was not enough money at the sale of the collateral to pay what was owed.
So, if they sue you, and get a judgment, then (in many states, etc.) they may be able to garnish your wages. Some states do not allow wage garnishments.
Consult with an Alabama consumer (or bankruptcy) lawyer, who will likely know the answers under Alabama law off the top of his or her head.