Well if you are feel bold enough about it to risk getting fired then ask him to show where your employment was based on having a car to be used for work purposes. Are they paying you mileage to use your car for business purposes?

Fact is if it is not clearly stated on your contract (and you did signe one) that you would have a car available for work purposes they have no leg to stand on but they are a corporation and can outspend you on lawyers. Might be able to get a lawyer to go for it with the potnetial of a major suit.