Box 14 is for information your employer wishes you to have, or is required to report to you by law, but which has no other place to be reported on the W-2. Personal use of a company vehicle is taxable income to you and is most likely already included in your W-2 wages. In this case, you will not need to enter it elsewhere.
I recommend you speak with your employer to confirm that this is how they handled your personal use before filing your return.
