After the repo occurs, the company would have to first file suit against you seeking the difference between the amount owed on the vehicle and the amount they recovered in a reasonable valued sale (this number may vary state to state). If they obtain a favorable judgment, wage garnishment is one of a number of legal tools commonly used to recover the judgment amount from a debtor.
There is no wage garnishment in SC, it is prohibited by law. They can repossess the vehicle but often times that results in claims you can bring against the auto-credit company. If your car is repossessed, you should contact an attorney about possible claims you may have as a result. Additionally, prior to any repo, the company must send you a right to cure notice setting forth how much is past due and what amount will bring your account current.