Some can gain a better understanding of this by looking at form 1040 to see how this flows. Here is a link to the 2017 form. The 2018 form will be different but the flow is the same. <a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/f1040.pdf">https://www.irs.gov/pub/irs-pdf/f1040.pdf</a>
Adjusted gross income is on line 38 of the 2017 form 1040. The standard or itemized deductions on line 40 are subtracted from adjusted gross income. The result is taxable income on line 43. (There are no personal exemptions on line 42 in 2018.) The tax tables or other calculations determine the amount of tax on line 44.
Non-refundable credits (meaning you must have tax to offset them in order to get them) are entered on lines 48-54. The electric vehicle credit would be entered on line 54. If there are no other credits and if the electric vehicle credit on line 54 is less than the tax on line 44, you are eligible for the entire electric vehicle credit to reduce the taxes. If the total of credits on lines 48-54 is more than the tax on line 44, the amount of credit you can get is limited by the tax on line 44.
Additions and subtractions on the remaining form determine how much refund you will get or taxes you will owe. One of those subtractions is the tax withholding (which your employer determines by the number of W-4 allowances you claim) entered on line 64 as a payment that is subtracted from the remaining tax owed. So tax withholding has no affect on how much credit you qualify for but it will affect the amount of refund or tax you owe when you file your tax return.