If your 2016 return has been filed and accepted, you would need to amend your 2016 return before you can claim the sales tax deduction.
Although the
sales tax from your car purchase is a deductible expense, but you must choose
to deduct either your state income taxes from 2016 (any income tax amount paid
from your 2015 state tax return and the state and local tax withheld in 2016)
or the sales tax from the car plus a standard sales tax deduction allowed by
the IRS.
The
IRS has a sales tax deduction calculator that can be helpful in determining
which deduction is the most beneficial.
IRS Sales Tax Deduction
Calculator