You amortize the total amount over the life of the plan. It does not matter if it is financed or not.
I don't think TurboTax has the option to do that within the vehicle section, so you will need to enter it separately under the depreciation section, or manually enter the yearly amount as a vehicle Repair&Maintenance.
To manually calculate the amortization you take the total cost and divide it by the number of months that the plan covers. For example, if it cost $600 and it was good for 60 months (5 years), you deduct $10 per month as a Repair/Maintenance.
