As you proceed through the questions in the "Business Vehicle Expense" section, you will come to a page at the very end that says "Before we wrap up, did you have any other expense for your [name of vehicle]?" The line for entering interest paid on the vehicle is on that page. That is the ONLY place in TurboTax that it should be claimed.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.