If you are talking about withdrawing money from a 401k or IRA, you will incur a 10% penalty unless you are over 59 1/2. If its an employer sponsored 401k, you may be able to borrow from the account if the plan allows for it. Check with your human resource department. Otherwise, premature withdrawals, (prior to age 59 1/2) are subject to the penalty, unless withdrawal is for hardship, (buying a car doesn't qualify). Hope this helps.

If you are still employed and under 59 1/2 you cannot just withdraw funds. You must be terminated, retirement age, or disabled to remove money -- the penalty applies to hardships under 59 1/2 (55 if terminated) unless it is for medical bills in excess of 7 1/2 percent of your gross income- and needing a car would not qualify.
Loan would be the only option if still employed, if your plan allows for them.