Are vehicle service contracts included in the vehicle cost basis used to calculate depreciation?

The service contract is a vehicle maintenance expense, not a vehicle purchase expense. The fact it may be included in the loan doesn't matter. So only the vehicle cost is depreciated over 5 years. But since it's all for business, that makes all the interest on the loan deductible, assuming the vehicle is 100% business use.

if you are an employee (get a W-2) your vehicle expenses are no longer deductible starting in 2018