You need to fill out an Insolvency Determination Worksheet. You can use either this one, or search for a fillable worksheet on the web.
You will need to determine the value of your assets and your liabilities on the day before your debt was forgiven. The amount of the debt forgiven counts as one of your liabilities.
You can look up the Kelley Blue Book value of your car on that date. You will also need the amount you owed for any other debts. For your assets, you will need the balance in your bank accounts, your investments, the furniture in your house and the clothes on your back. When estimating the value of your possessions, you can use the Goodwill Donation Valuation Guide. Don't sweat this last part - give it your best shot, and move on.