When you add your vehicle, under your self-employed business, and answer all of the question for the vehicle deduction, the software will calculate everything automatically. The deduction amount will show up under Vehicle Expenses on schedule C.
You won't be able to see all of the supporting schedules until after you've paid for your TurboTax account (under File tab Step #1). After you've paid, but before you've filed (under File tab Step #3), you can print your return to a PDF and review all of the supporting schedules. To do so:
https://ttlc.intuit.com/replies/3338601
Let me know if you have any follow up questions.
More Details on Entering Your Business Vehicle
You'll need to answer, when you "placed the vehicle in service" (started using this vehicle for business, which in your case means a prior year). If you answer 1 question incorrectly, it can take you down a different track and you won't be able to edit and change the answer. If you aren't getting the results you expected, add the vehicle again and delete your old entry. Re-considering and answering the questions, usually fixes this.
When you go back into the vehicle expense deduction; you'll see a summary screen listing the vehicles entered (because the deduction is by vehicle). Click Add another vehicle to enter your car again, Edit to review your entries and delete to remove your old entry.
