If the car allowance is not on your W-2, you may have an accountable plan, and if you do, you do not need to include the car allowance on your tax return.
According to IRS "If your employer reimbursed you or gave you an advance or allowance for your employee business expenses that's treated as paid under an accountable plan, the payment shouldn't appear as income on your Form W-2.pdf, Wage and Tax Statement. You don't include the payment in your income, and you may not deduct any of the reimbursed amounts."
https://www.irs.gov/taxtopics/tc514
