I have a vehicle that was used solely for business. It quit on me during the tax year, so it's unusable. When entering the basis gain/loss, how do I select loss?

TurboTax will calculate a gain or loss based on 1) the "sales" amount (zero if the vehicle is unusable) minus 2) your tax basis in the vehicle. 

If you have reported this vehicle in the past and used actual mileage, look for the Depreciation Report from last year, which will show you the original cost and the total depreciation you've claimed. Basis = Cost less Current and Prior Depreciation.

If you claimed the standard mileage deduction, you will need to calculate the Depreciation Equivalent based on the mileage you reported each year that you used the vehicle for your business. See this post for more info: What is "depreciation equivalent for total miles"