When you enter a car into TurboTax for business (or any tax software) it will ask your business use percentage (often based on mileage but sometimes just based on your word).
It will offer you the option to deduct 50% 0f the value of the car in that initial year - or maybe 100%.
Based on what you've written there is no way to tell what you've done with the depreciation on your return. I would recommend finding someone who understands depreciation and taxes to look at your returns over the last several years to figure out where you're at on the depreciation of the cars.