Ideally, I am guessing the person who has been allotted a vehicle for work use is either a senior personnel VP/Director or a Sales Relationship Manager. If you are looking for cool communication, then sending a letter is not. As you cannot send such 'memos' from the pages of your personal scrapbook. Yeah! No kidding.
If you send a letter, it has to be on the company's letterhead, and if it's just a normal memo, it has to be signed by a competent authority. We are talking about a violation of policy here. It can't be cool, and if it's cool, then it's not a violation.
I'd suggest, on a cooler note, send an email if it's allowed in your policy and state the reason and call for a meeting. Sending a letter is very, very official business. Consider you receiving one for late coming. How does that feel? Is this going to cost you your job? Is it that serious?
Besides, sending a letter means they are allowed to defend themselves, and that indirectly means you have the necessary proofs to accuse them in the first place. So, if you haven't yet jumped the gun, take a deep breath and choose your approach as per the intensity of the situation and the level of policy violation.
Best Wishes,
Erstine