A car allowance would typically be set up as a wage that does NOT contribute . Then you would have your usual deductions attached. These deductions would be taken out of your regular pay on the next check. So that means that in order to use this method, you need to still be paying one more check in the old year.
Also, when there is still one check to be issued in the old year, you might want to enter your excess life insurance premiums in the same method.
Either an earnings or a deduction can be set to go to box 14 or other W2 box numbers.