The electric car credit offsets the TOTAL amount of tax (Line 47 of 1040). Your withheld tax on your W-2 does not affect that at all.
The tax return calculates the TOTAL amount of income tax (Line 47 of Form 1040), not counting the taxes withheld from your paychecks (and other payments). From that amount, it makes some adjustments for credits and other taxes, then subtracts the amount of taxes withheld from your paychecks (and other payments) to determine if you get a refund or if you owe on your tax return. The Electric Car Credit is based on the first amount (before the adjustments and withholding), not the last amount (after the adjustments and withholding).
If Line 47 of your Form 1040 is at least $7500, you will get the full benefit of the Electric Car Credit, and your refund would increase $7500. Your refund or amount owed (before the Electric Car Credit) does not affect that.
