If you're entering actual expenses in TurboTax it will ask you to enter the cost of the vehicle when you purchased it, and it should ask you questions about prior depreciation you've taken. When you enter all the depreciation you've taken in the past (all of the prior depreciation should be shown on last year's tax return) TurboTax will be able to see that the vehicle is fully depreciated and not calculate a depreciation deduction this year.
